Abstract: Grass carp(Ctenopharyngodon idellus) is an important commercial fish species with great catches in China. Main stem of the Yangtze River is main reproduction ground of grass carp. Grass carp spends much of its time feeding in lakes where food is abundant and water is quiet . During spring ,when the water levels rise ,the adult fish migrate into the rivers to spawn. In June 2003 ,Three Gorges Reservoir began water storage and the water level reached 135 meters ,then up to 139 meters in November. It is worthy to be studied that whether the water storage will affect the hydrological conditions and sequently spawning time of grass carp in the middle reaches of the Yangtze River.The daily ages (D) of juvenile grass carps collected fromthe Dongting Lake and Xintankou section of the Yangtze River in July and August 2004 were determined by observing the daily increment ,and the hatch dates were estimated. The body length(BL,mm)growth equation of all samples was BL=-53.4195+3.1650D ,the weight(W,g) growth equation was W=019816 e0.0633D and the relationship between body length and body weight was W= 0.00001BL3.1003 .Estimated hatch dates primarily distributed between mid-May and early July. The earliest hatch date was 14-May ,which was a little later than the records before the water storage of Three Gorges Reservoir. It was thought that the hydrological conditions on the middle reaches of the Yangtze River might be changed after the water storage and then had affected on the hatch date of grass carp ,which should be proved in the further study.