Assemblages of forest insects across two high mountains(Mt.Hallasan:JJ and Mt.Jirisan:JR) in South Korea were compared by collecting insects using an ultraviolet light trap at 20 sites(200–1700 m elevation ranges) from May to October 2013.A total of 2960 individuals,representing 481 species of 10 orders,were collected on JJ,compared with 7080 individuals representing769 species of 14 orders on JR.The estimated number of species on JJ was 667 compared with 952 on JR.The differentiation among habitats(b-diversity) was higher on JJ(4.95) than JR(4.33) because of the island characteristics of JJ.Six insect orders(Lepidoptera,Coleoptera,Diptera,Hymenoptera,Hemiptera and Trichoptera) were dominant on both mountains,suggesting that the light trap captures represented well the insect fauna and is an effective method for investigating forest insect diversity.We concluded that forest insect assemblages on mountains are mainly affected by the elevation and the dominant forest in each elevation.In addition,the insect fauna on each mountain was differentiated by the habitat,which could be correlated with geological history. Assemblages of forest insects across two high mountains (Mt. Hallasan: JJ and Mt. Jirisan: JR) in South Korea were compared by collecting insects using an ultraviolet light trap at 20 sites (200-1700 m elevation ranges) from May to October 2013 . A total of 2960 individuals, representing 481 species of 10 orders, were collected on JJ, compared with 7080 individuals representing 769 species of 14 orders on JR. The estimated number of species on JJ was 667 compared with 952 on JR. The differentiation among habitats (b-diversity) was higher on JJ (4.95) than JR (4.33) because of the island characteristics of JJ.Six insect orders (Lepidoptera, Coleoptera, Diptera, Hymenoptera, Hemiptera and Trichoptera) were dominant on both mountains, suggesting that the light trap captures represented well the insect fauna and is an effective method for investigating forest insect diversity .We said that forest insect assemblages on mountains are primarily affected by the elevation and the dominant forest in each elevation. In addition, the insect fauna on each mountain was differentiated by the habitat, which could be correlated with geological history.