To clarify taxonomic status of the two sibling species, Moroco oxycephalus and M. lagowskii reproductive isolation mechanisms were investigated at sympatric area located in Kansung-up, Kosung-gun, Kangwon-do, Korea. Genetic analysis was performed to reveal mating system and intensity of Hybridization between the two species. The frequencies of hybrids were increased since 1989, and then the observed hybrid frequencies ($H_O$) did not significantly differ from the expected hybrid ($H_E$) in 1998 and 1999. However, based on histological analysis of two parents and their hybrid s gonads, the hybridizations between M. oxycephalus and M. lagowskii produced mostly fertile females but sterile males in accordance with Haldane s rule. Although it was suspected that pre- and postmating isolation mechanisms were affected between the two species, M. oxycephalus and M. lagowskii seemed to be strongly isolated with microhabitat at sympatry until 1997. Since 1998, hybrid frequencies were increased by habitat disturbance. However, their hybrid frequencies would be reduced by postmating isolation mechanisms. Therefore, the two species are considered to be distinct species recently diverged.