The acute toxic and biochemical effects of dichloromethane and 1,2-dichloroethane to Chlorella pyrenoidosa were assessed. The experimental results showed that dichloromethane and 1, 2-dichloroethane had some inhibition to growth of the algae. With increasing concentration of the organic toxicant, the cell density decreased. The 96 h-EC50 of dichloromethane to Chlorella pyrenoidosa was 550.1 mg/L; the 96 h-EC50 of 1,2-dichloroethane to Chlorella pyrenoidosa was 276.0 mg/L, toxicity of 1,2-dichloroethane is a little stronger than that of dichloromethane; the joint toxicity of two organic toxicants was showed as antagonism. Content of chlorophyll a, activities of superoxide dismutase (SOD), catalase (CAT) and content of malondialdehyde (MDA) in algae had been significantly affected under the organic toxicant exposure. The activities of SOD and CAT were increased at first and then decreased remarkably with the increasing concentration of the organic toxicant, showed as "Bell Shaped Curve". The content of malondialdehyde (MDA) flared up and lipid peroxidant aggregated with the increasing concentration of the organic toxicant. The results suggested that the primary mechanism of two organic toxicants on Chlorella pyrenoidosa cells may be related to oxidation damage of lipid and other biological large molecules caused by dichloromethane and 1,2-dichloroethane.