An annual investigation of Mollusca was carried out in Lake Taihu between November 2006 and October 2007. Altogether12 species were recorded, belonging to 12 genera of 9 families in 232 samples. The occurrence frequency of benthic Mollusca was90.0% in 30 sampling sites. The annual average density and biomass of Mollusca in its distribution area were 266ind./m², 102.2g/m²,respectively. Corbicula fluminea and Bellamya aeruginosa were the dominant species of Mollusca in Lake Taihu. The occurrence frequencyof C. fluminea was 90.0%, and the annual average density and biomass were 174ind./m², 58.3g/m², respectively, which mainlydistributed in the southwest part of Lake Taihu(393-896ind./m²) and Gonghu Bay (393-552ind./m²); the occurrence frequency of B.aeruginosa was 56.7%, and the annual average density and biomass were 58ind./m²,61.6g/m², respectively, which mainly distributedin the east part of Lake Taihu. Principal Components Analysis showed that habitat types and characteristics of sediment were the keyfactors determined the spatial distribution pattern of C. fluminea, while that of the gastropods was mainly influenced by the distribution ofmacrophytes, which coincided with the “mutualistic theory”. Analyzing the variation of Mollusca community structure between 1987and 2006, the results showed that the mainly distributed area of C. fluminea has changed from Meiliang Bay and Zhushan Bay to thecentral and southwest part of Lake Taihu, and the size of C. fluminea showed a tendency of decrease (P<0.001), while the averageweight of B. aeruginosa did not changed remarkably (P=0.051).