In the XIII century, the Lithuanian prince Mindovg united under his rule a part of the Lithuanian and Eastern Slavic lands, initiating the state, known as the Grand Duchy of Lithuania. Since 1392, the Principality of Polotsk (where princes of Lithuanian origin ruled since the 13th century) became an administrative unit within the Grand Duchy of Lithuania, governed by governors. The Grand Duchy of Lithuania reached its greatest territorial development in the second half of the 14th century, when its borders stretched from the Baltic to the Black Sea from north to south, from Brest to Smolensk from west to east. The secular-business language of the Grand Duchy was the Western Russian written language (in Belarusian historiography, the name is Old Belarusian, in Ukrainian historiography – Old Ukrainian).
Mid-XIII century in the state-building land, corresponding to the territory of modern Belarus, there have been dramatic changes. The lands of the Polotsk princedom (at that time large parts belonged to it) were incorporated into the Grand Duchy of Lithuania. There were many reasons for this gradual process, but the main targets were to create a political, military, and economic union between the Polotsk and Lithuanian lands against the crusaders, the Galician-Volyn princes, and the Tatar-Mongols. Finally Polotsk land became part of the Grand Duchy of Lithuania on the rights of autonomy, by agreement, in 1307. In 1320 the Vitebsk principality became part of the Grand Duchy of Lithuania.
The formation of the Grand Duchy of Lithuania was a long one. Dynastic marriages, agreements between separate principalities led to the emergence of a federative association. The leading role in it was played by ancient Russian forms of government with the relevant laws, language, and religion.
In 1508, a whole number of voivodships appeared in Belarus: Brest, Vitebsk, Minsk, Mstislavl, Novogrudok and Polotsk. Voivodships were divided into districts.
In the written language, the enlightener Francis Skaryna from Polotsk in 1517-1525, for the first time among the Eastern Slavs, began publishing activities. The vaults of legal documents – the Statutes of the Grand Duchy of Lithuania – were a classic example of the decorated feudal law of medieval Europe.
Since the end of the 15th century, the Grand Duchy of Lithuania, as a result of a series of wars, began to cede significant territories to the Russian state. The key battle of this period was the Vedrosha battle. A particularly difficult situation for the Lithuanian princes was formed in 1514, when the troops of Vasily III at the third attempt took Smolensk – a strategically important fortress, blocking the way deep into the principality.
The fate of the Belarusian lands in the Grand Duchy of Lithuania was greatly influenced by the transition of the Lithuanian nobility to Catholicism at the end of the 14th century. The Grand Duke Jagiello (1377–1392) in 1385 agreed to baptize the population of Lithuania according to the Catholic rite as one of the conditions for obtaining the throne of the Polish king ( Krevo union of 1385). From here onwards, the onset of internal tension between the Orthodox and the Catholic population is brewing. Since the adoption of the Gorodelsky Privilege in 1413, the political situation in the GDL began to be characterized by the dominance of the Catholic nobility, which received the exclusive right to hold public office and the title of gentlemen. East Slavic boyars and princes retained their participation in the affairs of the Grand Duchy of Lithuania, they supported the state, contributed to the growth of its power, and also sought to increase their influence on political decision-making. The expression of their interests was the Grand Duke Švitrigaila, who began distributing government posts, castles and parishes to Orthodox Russians (Belarusians, Ukrainians) and feudal lords, which led to a long feudal war ( Civil War in the Grand Duchy of Lithuania 1432-1438 ). In 1432, Švitrigaila fled to Polotsk, where Belarusian princes and boyars put him on the throne of the Grand Duchy of Russia, thus a brief ON split occurred, and Polotsk became for some time the capital of the Belarusian state. The Catholic leaders of the Great Patriarchate compromised. Under the Grand Duke Casimir IV, separate lands (Novogrudskaya, Polotsk, Vitebsk, Smolensk, etc.) the rights to autonomous administration were confirmed. Casimir approved the lawsuit in 1468 – the first ordered set of laws. After his death, the Moscow State began the war for the unification of the lands of Ancient (Kiev) Rus. In the course of these wars to Muscovy in the 15th and 16th centuries, some East Slavic lands of the Great Lithuanian League were annexed. After the failures in the wars, the political leaders of the Grand Duchy of Lithuania began to involve large Orthodox feudalavs in a more active administration of the state; Orthodox were in fact no longer forbidden to hold important government posts. In 1563, a provision was legally abolished, of which the Orthodox fevdals could not sit on the council of the masters. The magnates of Ostrog, Chodkiewicz, Sapegi, Illinich, and others had a great influence on state affairs.
By the middle of the XVI century. the state structure of the Grand Duchy of Lithuania was finally formed, the foundations of which were enshrined in the Statute of the Grand Duchy of Lithuania in 1529 and the Statute of the Grand Duchy of Lithuania in 1566. The state language of the Grand Duchy of Lithuania was Old Belarusian. The head of state was considered the Grand Duke (master). Together with the council, he carried out the highest court. Originated in the XV century as an advisory body under the Grand Duke, and by the end of the 15th to the beginning of the 16th century, it became the highest state authority. In 1413, the territory of the Grand Duchy of Lithuania was divided into the Vilno and Trokskoe voivodships. At the beginning of the XVI century the Polotsk, Novogrudok, Smolensk and Podlaskie voivodeships formed; as a result of the administrative-territorial reform of 1565–1566 – the Brest, Minsk, Mstislavl voivodships, some of them were divided into counties (Administrative-territorial division of the Grand Duchy of Lithuania). The Belarusian nobility received the right to participate in a seismic, where ambassadors (deputies) were elected to the free parliament. The Livonian War of 1558–1583 made a great influence on the fate of the GDL.