Species turnover and temporal variation of forest insects were used to explain the ecological succession and ecological segregation between efficiently competing species. In this study, species richness, abundance, and beta-diversity of the genus Onthophagus (Coleoptera: Scarabaeidae) assemblages between 2003 and 2007 were described and the diel—flight activity was examined in the disturbed forest and the interior forest of the lowland tropical rain forest at Ton Nga Chang Wildlife Sanctuary in peninsular Thailand. A total of 2,260 individuals of 22 species in 2003 and 2,382 individuals of 24 species in 2007 were collected. Although species richness and abundance did not differ significantly between the two years, all similarity indices were significantly different. The community structure of Onthophagus assemblage in 2003 demonstrated a heterogeneous pattern, whereas there was a tendency for the pattern to shift toward a more homogeneous structure in 2007. The temporal variation showed two distinct diel—flight activities; diurnal and crepuscular patterns. Six species were crepuscular (O. deflexicollis Lansberge, O. orientalis Harold, O. rudis Sharp, O. sp 1, O. sp 2, and O. sp 4), whereas most of Onthophagus species demonstrated diurnal pattern. Remarkably, five species (O. taurinus White, O. pilularius Lansberge, O. punneeae Masumoto, O. laevis Harold, and O. sp 3.) could not be classified as either diurnal or crepuscular species. It was suggested that the species turnover was probably influenced by the recovery of the forest structure and the decrease of anthropogenic disturbance. Resource partitioning was suggested to be a key factor for crepuscular adaptation in Onthophagus species.