Copper sulphate treatment is widely used as a global and empirical method to remove or control phytoplankton blooms. We tested the acute toxicity of 5 species of familiar microalgae and gave an urban lake copper sulfate treat in order to illuminate how the water body response to copper treat, especially the change of phytoplankton species and density of microcystins. According to OECD Alga growth inhibition test, effect of copper on 5 species of familiar microalgae was tested, and species of Cyanophyta showed much higher sensitivity to copper than species of Chlorophyta and Bacillariophyta. In a eutrophic urban lake, CuSO4 center dot 5H(2)O was used as algicide to control the water bloom caused by cyanobacterial. The copper concentration applied was 102 mu g/L ( as copper). We investigated the lake response to copper sulfate I which showed that the transparence was substantially improved; TN and TP did not change a lot; total algal decreased just after the copper sulfate treatment, and later increased again; at the beginning of the experiment, species of Cyanophyta died and the amount sharply decreased, Bacillariophyta and Chlorophyta turned to be the preponderant species, which could be related to species sensitivity to copper, and species of Cyanophyta began to grow again and took the predominance few days later; density of microcystins in the water rapidly decreased to level lower than that in the treatment before, only in 4d.