Abstract: LY -04 andWSQIF are newly -synthesized compounds whichwere presented by the Institute of PesticidesChemistry, Central China Normal University. LY -04 belongs to -oxophosphonic acid derivatives, while WSQIF is an aromatic acetyl oxophosphonic acid derivative. In this study, the toxicity of LY -04 and WSQIF on Scenedesmusobliquus Kǜtzwas examined. The EC50of LY 04 andWSQIFwere determined in S.obliquus cells at 24h, 48h, 72h, 96h representedby celldensity. Our data showedthat the 24h, 48h, 72h, 96h EC50of LY -04 were 1983mg/ L, 2943mg/L, 240mg/ L and 65. 0mg/ L acetone, respectively, and the 24h, 48h, 72h, 96h EC50of WSQIF were 15. 9mg/L, 53. 0mg/L, 98. 0mg/L and 28. 8mg/ L respectively. LY -04 displayed a higher insecticidal activity, while WSQIF showed a higher phytocidal activity. As LY -04 and WSQIF were designed to act as insecticide and herbicide, respectively, the results were in contrast to the anticipated ones. According to the toxicant classification standard, EC50of both substanceswas far higher than 3mg/ L, while the toxicity of LY -04 and WSQIF to aquatic algae were fairly low, indicating that these two pesticides are rather safe to the aquatic ecosystem.