The annual dynamics of Brachionus (Rotifera) populations in Lake Jinghu of Anhui Province, China was studied in the year 2003, and the relationships between their body volumes, egg volumes as well as relative egg volumes and the ecological factors including water temperature, Chl-a concentration were regressively analyzed. Seven Brachionus species in Lake Jinghu were found, of which Brachionus calyciflorus, B.angularis, B.diversicornis and B.forficula were determined as common species due to their higher population densities and occurrence frequency. The population density of both B. calyciflorus and B. angularis reached the peak in February and March and decreased rapidly after April, while that of both B. diversicornis and B. forficula remained high during June-September and decreased sharply after October. The body volumes of the four common Brachionus species excluding B. forficula were significantly correlated to water temperature, and their mean body size was positively correlated with their mean egg volume. The body volumes of B. calyciflorus and B. angularis were both negatively correlated with Chl-a concentration, and their egg volumes were negatively associated with water temperature and Chl-a concentration, respectively, with water temperature being the greatest effect. The relative egg volume of only B. angularis was positively correlated with water temperature.