Abstract: To determinate the species composition, distribution and resources condition of mollusca of Xiang River trunk stream and to probe into the effect of dominant species, endemic species, rare species and ecological factors on mollusca of Xiang River, a systematic investigation of species composition, distribution and population size of mollusca from 36 samples of 15 counties in Xiang River was conducted from 2005 to 2006. Diversity indices of mollusa from 12 sample sites related to influence factors were calculated.The results showed that a total of 72 species belonging to 29 genera in 10 families of 2 classes were found. Among them, 38 species belonged to Gastropoda and 34 belonged to Bivalvia. Ten species were first reported in Hunan province and 42 species were endemic species in China. One species, Mekonia hunanensis, which has been listed in the China Species Red List as an extinct species, was first found in the recent 60 years. The mollusca of Xiang River was in Oriental realm and had obviously subtropical characteristics. There were 6 dominant species of mollusca, i.e. Bellamya purificata, B. aeruginosa, Parafossarulus eximius, Semisulcospira gredleri, Limnoperna lacustris and Unio douglasiae. There were 13 rare species such as Filopaludina bengalensis, Bellarnya costata, B. smithi, Mekongia hunanensi, Angulyagra wilhelmi and so on. The species composition and population size of mollusca at upper, middle and lower reaches of Xiang River had obvious differences. The species number of Gastropoda was similar at each river section, while the species number of Bivalvia showed great difference at upper, middle and lower reaches as 12, 22 and 32, respectively. In terms of the population size, Rivularia, Semisulcospira cancellata, Corbicula fluminea and Corbicula largiUierti saw a gradual increase from upper to lower reaches, while Semisulcospira gredleri and Corbicula nitens saw a gradual decrease. The main factors affecting mollusca of Xiang River were town sewage, hydro-electric engineering and sand mining. Moderate amount of sewage, which was discharged into the river by cities along the river, can properly increase the species composition and quantity of mollusca. On the other hand, a great deal of sewage and industrial effluent, which was discharged into the river without treatment will cause the water pollution and in turn lead to an obvious fall in the species composition and the quantity of the mollusca. Moreover, the establishment of reservoirs to conserve water at upper and middle reaches had a great impact on mollusca in the reservoir area. The rising of water level and the environment upheaval made the species composition and the quantity of the mollusca, especially the Bivalvia, reduce significantly. The storage of water at upper and middle reaches, which made dry season longer at lower reaches and the water quality of waterfront worsened or even dried up, has caused a great number of mollusca death and seriously affected the population size. In addition, the development of sand mining, which has made river bed deepen and turbidity degree of water body increased, together with the like-oil compounds which usually polluted water bodies, affected the normal growth of mollusca or even suffocated the mollusca, and finally led to a decrease in the population size.