Abstract: 1-［hydroxyl-diethyl-malonate］-isopropyl dodecenoic acid (C₂₂H₃₈O₇), a type of growth-inhibitor, was successfully isolated and purified from cultural liquid of Isochrysis galbana. A study was conducted to investigate the inhibitory effects of HDMA on the growth of Isochrysis galbana. Results showed that exogenously added HDMA inhibited the growth of this algae. HDMA significantly decreased the chlorophyll contents of algal cells. And it had a very significant effect on the ratio of extracellular protein to polysaccharide and activities of NR (nitrate reductase), SOD (superoxide dismutase) and POD (peroxidase) of algal cells. But it was obvious to increase MDA (malonyldialdehyde) contents of algal cells. The results gave hints to elucidate the species-specific antialgal mechanisms of HDMA. Inhibition mechanism of HDMA on Isochrysis galbana is to decrease the chlorophyll contents, and decrease or increase activities of NR, SOD and POD of algal cells, and change the ratio of extracellular protein to polysaccharide, thus increasing ratio of extracellular protein to polysaccharide leads to the increase in hydrophobicity of algal cells, finally algal cells flocculate and subside. Affecting uptake of inorganic nitrogen by cells and photosynthesis might play a major role in the inhibitory mechanism. And the excess active oxygen was produced under HDMA stress and active oxygen participated in the damage of HDMA to this algae. It indicated that the physiological metabolism of Isochrysis galbana was inhibited by HDMA.