Yakovlev belongs to the advanced researchers of the history of Ukrainian law. Of the works of Yakovliv (Ukrainian, Russian, French, German and English), the most important: "Agreement of Hetman B. Khmelnitsky with Moscow in 1654" (1927), "On Kopny Courts in Ukraine" (1931), "Moscow Projects of Contract Clauses with Hetman I. Vygovsky" (ZNTSH, 1933), "Ukrainian-Moscow Agreements 17 -18 centuries." (1934), "Fundamentals of the Constitution of the UPR" (1935), "Das deutsche Recht in der Ukraine" (1942), "Ukrainian Code 1743" (1949), "Treaty of B. Khmelnitsky" (1954). In a number of works devoted to Russian-Ukrainian relations 17. Yakovlev defends the view that the Pereyaslav agreement formed a kind of vassal relationship of the protectorate type between Russia and Ukraine.