Coilia nasus is a dominant species in the fish community of Dianshan Lake and investigation on its diet composition contributes to understanding interspecific relationship and ecosystem structure of this lake. A total of 387 stomach specimens of C. nasus were collected in Dianshan Lake from August 2009 to July 2010. Based on those specimens, the diet composition of this fish was conducted using the traditional method of stomach contents. C. nasus preyed all year round with the highest index of fullness in summer and the lowest in winter, and with the highest rate of empty stomach in antumn and the lowest in spring. The diet composition of C. nasus covered 13 large and 30 small categories. Cluster analysis showed that numerical percentage (N%), occurrance rate (F%), and percent index of relative importance (IRI%) are consistent in measuring importance of each food category to C. nasus, all indicating that Cladocera and Copepoda were the main food sources for C. nasus; However, there were significant differences in measuring the contribution between gravimetric percentage (W%) and each of the three above mentioned indices. The W% displayed that the shrimp, Cladocera, and fish were the main food sources for C. nasus. The N% of the food composition of C. nasus did not significantly differ among seasons, but the W% of food composition did. Combining the results of all the four indices, we concluded that C. nasus in Dianshan Lake mainly feeds on crustacean zooplankton, fish and shrimp. The information on diets of C. nasus suggests that this fish may promote the growth of phytoplankton by top-down effect, implying that reduction of C. nasus' biomass would contribute to the ecological restoration of Dianshan Lake.