Allelopathic effects of Ulva prolifera on the harmful algae (Heterosigma akashiwo) were studied in laboratory. The results showed that the growth of H. akashiwo was significantly (P>0.05) lowered by fresh tissue of U. prolifera. The growth of H. akashiwo was not significantly (P>0.05) lowered by U. prolifera culture medium filtrate under initial filtrate addition, but the growth of H. akashiwo was strongly inhibited by macroalga culture medium filtrate under semi-continuous filtrate addition. The growth of H. akashiwo was strongly inhibited by dry powder and methanol extract of U. prolifera. The results showed that allelochemicals existed more in the tissue of U. prolifera than excreted to the medium and continuous addition of allelochemicals. It was important to effectively control the growth of H. akashiwo. The concentration effect of antagonistic relationship existed between U. prolifera and H. akashiwo.