Enteromorph prolifera was the causative species of green tides in the Yellow Sea from 2007 to 2009. Three different organic solvents which were methanol, acetone and chloroform, were used to extract the dry tissue of , and the effects of extracts on hatchability of Pacific oyster) eggs were tested. It was found that the methanol and acetone extracts had strong inhibitory effects on hatching of oyster eggs, and their high-concentration extracts could completely inhibit the oyster eggs from developing into D-shaped larvae. The chloroform extract, however, did not show any significant concentration-dependent effects on the hatching rates of oyster eggs. The methanol extract of was further purified by liquid-liquid extraction with ethyl acetate and petroleum ether, and the ethyl acetate phase and petroleum ether phase were also tested for their effects on the hatching rates of oyster eggs. It was found that both ethyl acetate and petroleum ether extract could significantly inhibit hatching of oyster eggs. The results of this experiment suggest that can produce bioactive compounds inhibiting hatching of oyster eggs, and the compounds, probably with high polarity, can be extracted with methanol. However, the structure and characteristics of the bioactive compounds in need to be further studied.