The aim of the article is to present changes in the position of the Grand Duchy of Lithuania towards the Polish Crown within the Polish-Lithuanian Commonwealth in the years 1569–1791. The analysis was based on the transformation of both common (monarch, Sejm) and separate (central and local offices, judicial system and law) institutions in the context of economic, social and cultural changes of the era. Gradually, the Commonwealth was transforming into a state in which Lithuania was not so much one of its two parts – along with the Crown, but one of its three provinces – along with Lesser Poland and Greater Poland. It was, however, a special province since it had its own ministers, offices, courts, treasury and fiscal courts along with its own codification of political and private law. The rule introduced in 1673 that every third Sejm was to be held in Grodno, however, was rarely observed. The reasons for this change were: the smaller population of the Grand Duchy, its lower fiscal income along with the war damage and territorial losses suffered in the mid-17th century. This transformation was also facilitated by the fact that the Lithuanian political system and laws became increasingly similar to the Polish ones. Another factor was the slow creation of a sense of political community among nobles of both the Crown and Lithuania. This feeling was born not only out of the same rights and privileges, but also owing to the immigration of the Crown noblemen to the lands of the Grand Duchy and joining – by marriage – Lithuanian noble families, which was especially the case among magnate families. During this period, the common culture of the Polish-Lithuanian Commonwealth based on the Polish language – lingua franca of the whole state – was also created. Other languages also functioned in the Polish- -Lithuanian state, but Polish, enriched by Lithuanian, Ruthenian, Latin, German and Oriental elements, began to dominate. The Government Act of 3 May 1791 did not mention the Grand Duchy at all, but created a common government for the whole of Commonwealth – the Guard of Laws and Great Commissions. Mutual Pledge of the Two Nations, unanimously adopted on 20 October 1791, constituted an attempt to return to the dualism from the era of the Union of Lublin. This act granted Lithuanians half of the commissioners in the military and fiscal commissions and – in the future – in the police commission. Lithuania also retained separate ministers, offices, a separate treasury and tax judiciary. Thus, the gradual unification of the Polish-Lithuanian Commonwealth was still visible, while maintaining some separate aspects, which were important for Lithuanians, albeit secondary in the scale of the entire state. Nevertheless, this process was interrupted by the upcoming partitions.