In this paper, the experimental validation on inhibitory effects between Ulva prolifera , a popular specie of green tides in the Yellow Sea, and Isochrysis galbana under controlled laboratory conditions, and the possible mechanism was studied. In the coculture system with 〖WTBX〗Ulva prolifera〖WTBZ〗 at 1 gFW·L ^-1 and Isochrysis galbana at 1×10 ⁴ cells/ml, the concentration of NO₃^--N and PO₄^3--P were significantly higher in the cocultured medium than that in the monocultured medium of U.prolifera and I.galbana for initial 3 days. It indicated there was inhibitory effect between Ulva prolifera and Isochrysis galbana, and the inhibitory rate for U.prolifera to I.Galbana was 96.62%, while the inhibitory rate for I.Galbana to U.prolifera was 19.67%. It resulted in decrease of uptake rate of U.prolifera and I.Galbana for nutrients. From the fourth day to the end, the concentratons of NO₃^--N and PO₄^3--P were gradually increased, and the cell breakdown, swollen and death of U.prolifera and I.Galban were observed under microscope. The concentration of NO₃^--N and PO₄^3--P in the monocultured medium was constantly decreased. It indicated that U.prolifera and I.galbana released nutrients into the seawater. It demonstrated that there was the allelopathic effect between U.prolifera and I.galbana when the nutrients in seawater and the biomass of U.Prolifera were relatively low.