The block cipher GOST was proposed in former Soviet Union in 1989. In this paper we present the first result of differential cryptanalysis of GOST with reduced number of rounds. By introducing the idea of using a set of differential characteristics, which is a partitioning type, we can reduce the influence of the key value upon the probability as well as get high differential probability. Using 2⁵¹ chosen plaintexts the key of 13-round GOST can be obtained. Next this differential cryptanalysis is expanded with combining related-key attack. Using 2⁵⁶ chosen plaintexts the key of 21 rounds of GOST can be obtained.