A standard method of algal bioassay for evaluating the toxicity of toxic chemicals was applied in the study. It was found that safety concentrations of Cu, Zn and Mn for Chlorella pyrenoidosa's growth were 31.8 micrograms/L, 65.0 micrograms/L and 5.5 mg/L, and 96 h-EC50 were 67.3 micrograms/L, 473.0 micrograms/L and 17.0 mg/L, respectively. According to both safety concentration and 96 h-EC50, the toxic sequences of these metals from high to low on the inhibition of C. pyrenoidosa's growth were Cu, Zn and Mn. The different affinities between different metal ions and algal cells are mainly responsible for the different inhibitions of C. pyrenoidosa's growth.