As an important component of biodiversity, the distribution pattern and the determinants of insect diversity has attracted increasing attention. To explore the distribution of insect species richness along the altitudinal gradient in the Three Gorges Reservoir Region, we conducted a series of analyses on a database of 3517 insect species recorded within this area. By compiling the altitudinal range of these species, including the herbivore pest insects, we explore the altitudinal patterns of insect diversity of different taxonomic levels and particular families, and by different range size classes. Regression tree modeling was then applied to uncover the prominent factors regulating the altitudinal patterns of species richness of total insects and pest insects. The analysis revealed that: 1) a unimodal richness pattern along the elevation gradient was found consistently for different systematic levels. Although the patterns are significantly different for the species belonging to the restricted, middle, and broad range size classes, the species richness of all three classes decrease prominently above 1400m a.s.l., whereas the species richness of pest insect begins to decline from 1200 m a.s.l. upwards; 2) TWINSPAN was applied and classified the insect families into 4 modes with different altitudinal patterns of species richness. Although the estimates are biased because of greatly unbalanced sampling, all the four modes are unimodal type; (3) the change of land cover composition along the altitudinal gradient plays a direct and primary role in regulating the patterns of all insects and pest insects species richness. Water-energy or primary productivity indicated by the actual evapotranspiration follows as a prominent climatic variable for the pattern of insect species richness. The inflection point of species richness is probably controlled by the isoline of 0℃ in January, representing the threshold temperature for the upper limit of the majority of insect species. We suggest that the altitudinal patterns of insect species richness, as well as their taxonomic components are the result of joint effects of human activity and climate variation along the gradient, with the former being more direct, reflected as the altitudinal change of land cover compositions. This would have substantial implications for insect diversity resource management, as well as the preventive treatment of pests in agricultural-forestry ecosystem.