Abstract: The toxicological response of the freshwater green alga Selenastrum capricornuium to cadmium, as well as uptake and accumulation capacity. has been studied in this paper. The results show S. capricornutum response issensitive to the toxic action of cadmium and is suitable for toxicity test of cadmium. After 96-hr exposure, the EC₅₀ is. found to be 0.83 mg mg/l. the lethal concentration is 2ppm. In the range of sublethal concentration cadmium (0.25-1.5ppm), as the concentration of cd is increasing, the contents of DNA and chlorophyll a of S. capricornutum reduse evidently, its deoxyri-bonuclease activity, dehydrogenase activity and peroxidase activity are affected intensely. its cell division oxygen evolution and cell membrane permeability are also inhibited and destroyed significantly.