A standard test method was applied to evaluate the toxicity of chemicals to algae;and inhibitory effects of 96 h median effective concentration (96 h-EC50) of eight kinds of commonly used dyes on the growth of Scenedesmus obliquus were calculated using the method of straight-line interpolation for data treatment. It was found that alkaline dyes had more inhibitory effects on the growth of Scenedesmus obliquus than active dyes. It can be concluded from the results that the chlorophyll-a growth rate of Scenedesmus obliquus was more sensitive than the cell growth rate,and they had the same sequence in reaction of the dye's toxicity.