The study was investigated lepidopterous insect fauna of Mt. Bokju-san located in Cheorwon-gun, Gangwon-do, Korea, its changes in the number of monthly species and the number of individuals from April, 2011 to March, 2012. This study was identified a total of 401 species belonging to 16 families of lepidopterous insect fauna inhabiting Mt. Bokju-san. Based on dominance by families, Noctuidae, with 133 species, was the most dominant species, followed by Geometridae (115), Crambidae (32), and Notodontidae (25). The most abundant species was Cosmia camptostigma (214 individuals), followed by Catocala streckeri, Arichanna melanaria, Bombyx shini. Monthly distribution indicated that Geometridae was the most dominant species from November to May, and Noctuidae was the most dominant from June to October. The number of individuals increased from March to June. On the other hand, it decreased consistently after June. The number of collected individuals showed that Noctuidae (834) was the largest of all, followed by Geometridae (748), Arctiidae (164). With regard to the dominant species in early spring, Apochima juglansiaria and Conistra fletcheri were dominant in March and Idiotephria amelia in April. Inurois brunneus was dominant in late fall.