Legal formation is one of the basic categories of both historical and legal science and legal science in general. The law-making process is a concentrated reflection of legal values, legal interests, legal mentality and mentality, ultimately the sense of right of society. His analysis is important for the reproduction of the historical and legal reality, particularly in the Ukrainian lands of the 15th-17th centuries, when legal practices and strategies were established and the legal canons of Ukrainian society were established. In this connection, the study of the organization of Diets and Diets and their functioning has significant cognitive potential. The purpose of this article is to highlight some features of the law-making process on Ukrainian lands in the relevant period, which manifested itself in the phenomenon of seymikation – participation in Sejms and sejmiks. The appearance of noble self-government bodies was a consequence of the spread of the crown system of administrative-territorial and judicial system to the territory of Ukraine. As a result of the administrative reform carried out on the eve of the Union of Lublin and the division of lands into voivodeships and counties, Ukrainian lands became part of the Polish-Lithuanian Commonwealth as part of 8 voivodeships, with all the rights of the ancient crown voivodships and at the same time with «their own rights». Such «own right» was primarily the Lithuanian Statute of 1566 (Statute of the Grand Duchy of Lithuania) or the so-called Volyn statute. It was he who introduced the two-tier system of state organization: the general Sejm – the place of state representation of regional elites at the national level, and the poviat sejmiks – representative bodies of the nobility. Participation in seyms became the main mechanism for the participation of the nobility in the political and legal life of the state. Despite all the specifics of sejmik (vassal-clan dependencies, tycoon lobby, etc.), it can be argued that it, as well as the institutions of the Sejm and sejmiks, ensuring the involvement of a significant nobility and communication between the lord and the nobility, was an effective way of organizing, in particular, rightful life, the main component of which is law-making. Key words: legal formation, Sejm, sejmiks, sejmіkаtion, Lithuanian statute, nobility, crown law, selfgovernment, law.