Abstract: The effects of ionic liquid 1-hexyl-3-methylimidazolium (C₆mimBr) at the concentrations of 1,5,10,15 and 20 mg·L^-1 on antioxidant enzyme activities,lipid peroxidation of Scenedesmus obliquus were determined in the present study after treated for 24,48 and 96 hours,respectively.The results indicate:After exposed for 24 hours in C₆mimBr,the contents of soluble protein and malondialdehyde (MDA),the activities of superoxide dismutase (SOD) and catalase (CAT) increased significantly at all the concentrations.At the 48th hour and 96th hour SOD activities were at higher level.Its peak values appeared at the lower concentration of C₆mimBr.The change trends of the activities of CAT,the contents of soluble protein and MDA were increasing with the increasing of the concentration of C₆mimBr at the 48th hour and 96th hour,but compared with the data at the 24th hour,most of them decreased a little.The change trends of the CAT activities had great similarities with that of the contents of soluble protein.Both their change trends were also similar to that of MDA content.The C₆mimBr induced the lipid peroxidation of S.obliquus and had environmental toxicity.SOD and CAT could be used as sensitive biomarkers at the molecule level of the stress of C₆mimBr.