The inhibitory effects of growth-inhibitor on the growth of Isochrysis galbana, malondialdehyde (MDA) content and activities of antioxidant enzymes (superoxide dismutase and peroxidase) were studied. And the inhibition effects of 4 species of antioxidants on this damage to its cells were also investigated. The results showed that 0.10 mg/L of GI marked decreased cell densities, activities of superoxide dismutase (SOD) and peroxidase (POD), and MDA content increased. With increase of GI concentrations, cell densities and activities of SOD and POD decreased sharply, and MDA content increased obviously. When concentration of GI was 0.30 mg/L, the cell densities, activities of SOD and POD decreased to 0.05, 0.56 and 0.59 folds of the control, respectively, and MDA content increased to as higher as 2.2 folds of that of the control set. L-Ascorbic acid, citrazinic acid, lenediamiaetetraacetic acid disodium salt (Na₂EDTA) and 3-tert-butyl-4-hydroxyanisole could decrease MDA content obviously, and marked increased cell densities and activities of SOD and POD. When 4 species of antioxidants were added to the medium, the cell densities, and activities of SOD and POD increased to as high as 1.38～1.90, 1.49～2.12 and 1.55～2.13 folds of that of the control set, respectively, and MDA content decreased by 57.7%～87.9%. It was indicated that the excess active oxygens were produced under growth-inhibitor stress and active oxygen participated in the damage of growth-inhibitor to Isochrysis galbana; the L-ascorbic acid, citrazinic acid, lenediamiaetetraacetic acid disodium salt and 3-tert-butyl-4-hydroxyanisole could effectively inhibit membrane lipid peroxidation and further alleviated algal damage.