Abstract: HNPC-C99082-(4-methoxy-6-methylthiopyrimidin-2-yl) carbamoyl sulfonyl benzoate is a novel sulfonylurea herbicide discovered by Hunan Branch of the National Pesticide R&D South Center, Changsha, China, which is effective in controlling various broadleaf weeds and some grasses in the wheat field. Toxicity and toxic mechanism of HNPC-C9908 to the green algae, Chlorella pyrenoidosa Chick, were studied in the laboratory. The experimental results showed that HNPC-C9908 had obvious effects on the growth of the algae, depending upon the concentrations. The growth stimulation was observed under the low concentration of 1 mg/L HNPC-C9908, and the obvious inhibition of the growth of the green algae under the high concentration of more than 25 mg/L. Its 96h-EC50 value of HNPC-C9908 against the algae growth was 29.12 mg/L, indicating that HNPC-C9908 was considered as a low toxic pesticide to the green algae. Meanwhile, HNPC-C9908 also exhibited some inhibitory influences on the photosynthesis pigments such as chlorophyll a and b. The chlorophyll content of the algae cells declined with the increasing concentration of HNPC-C9908, showing the good concentration-effect relationship. In the presence of HNPC-C9908, the soluble protein content, the two key enzymes responsible for eliminating the active oxygen superoxide dismutase (SOD) and peroxidase (POD) activity in the algae cells exhibited the obvious changes. The irritable increases of the protein content, SOD and POD activity in the algae cells were observed under the low concentration, and significant inhibitions appeared under the high concentration with the 96 h-IC50 values of 26.27 mg/L, 15.25 mg/L and 13.76 mg/L, respectively, demonstrating that HNPC-C9908 resulted in the decline of SOD and POD activity in the algae cells, making the active oxygen excessive and accumulative, algae cell membrane lipid peroxidation, and thus caused the damage of the algae cells.