Abstract: LY-04 and WSQIF were two newly-synthesized compounds which were presented by the Institute of Pesticides Chemistry, Central China Normal University. LY-04 belonged to α-oxophosphonic acid derivatives, while WSQIF was aromatic acetyl oxophosphonic acid derivatives. In this paper, the toxicity of LY-04 and WSQIF on Scenedesmus obliquus Kütz. were studied. The 24 h, 48 h, 72 h, 96 h EC₅₀ of LY-04 and WSQIF in acetone were determined by detecting the cell density of the S. obliquus cells. It indicated that the 48 h, 72 h, 96 h EC₅₀ of LY-04 were 2942.5 mg/L, 239.7 mg/L and 65.0 mg/L, respectively, and the 24 h, 48 h, 72 h, 96 h EC₅₀ of WSQIF were 15.9 mg/L, 53.0 mg/L, 98.0 mg/L and 28.8 mg/L, respectively. A series of (studies) showed that LY-04 had higher insecticidal activity, while WSQIF had higher phytocidal activity. As LY-04 and WSQIF were designed for insecticide and herbicide respectively, the results were in compete with the anticipated ones. And according to the toxicant classification standard, EC₅₀ of LY-04 and WSQIF were all far higher than 3 mg/L, therefore the toxicity of LY-04 and WSQIF to the aquatic algae were low, indicating these two kinds of pesticides were rather safe to the aquatic ecosystem.