The insects were investigated from March 2004 to November 2004 at Zhifanggou watershed in hilly area of loess plateau..The result showed that the insect species varied spatially in the following order：the natural bush forests，the mixed forests，and the simple forests.The number of the insect individuals in Caragana korshinskii were the highest，followed by mixed forests and Hippaphae rhamnoides，and the number of the insect individuals in the natural bush forests were the smallest.The season changes of number of the insect species and individuals in various vegetations can be described by a function y=ax3+bx2+cx+d.The order of diversity value of the insect community in various vegetations from May to September were the natural bush forests， the mixed forests and the simple forests.The evenness value’s change range of the simple forests and mixed forests are higher，and the natural bush forests are smaller.Contrary trend are assumed in the dominance values and the evenness values in time course.The principal component analysis were used for studying main factors and the temporal patterns of the insect communities in different time， and the insect commuonty in the natural bush forests had a more complicated composition，less fluctuation，more obvious mian factors and temporal patterns.However，in the mixed forests and the simple forests，the composition of the insect commuonty were simple and fluctuated greatly，their mian factors and temporal patterns were not obvious.