The effectsof ionic liquid 1-hexyl-3-methylimidazolium (C6mim Br), with the concentrationsof 0, 1, 5, 10, 15 and 20 mg/L and exposed for the 24, 48, 72 and 96h respectively, on glutathione (GSH) and the enzymes in glutathione metabolism of Scenedesmus obliquus such as glutathione peroxidase (GPX), glutathione S-transferases (GST) and glutathione reductase (GR) were studied in the present study1 The results indicated that the GSH contents decreased with the concentration of C6mim Br increasing at the 24, 48 and 72h, but it did not change when C6mim Br = 1mg/L. The GSH contentswere similar to or lower than that of the control level at the 96h. The activities of GPX and GST increased markedly before 72h except for the activity of GST at the highest concentration (20 mg/L), but they both decreased to the control level at the 96h1 The activities of GR increased at the concentration of 1 mg/L and then decreased with the concentration increasing at the 24h, increased to the control level at the 48h, and then reached its peak at the 72h (when C6mim Br≥10 mg/L the GR activities exceeded the control level), but at the 96h, it decreased markedly even weaker than the control level. GR was the enzyme that limited the reaction velocity in the system of glutathione cycle and the GST was the most active and sensitive enzyme. It could be used as sensitive biomarkers under the stress induced by C6mimBr. C6mim Br induced oxidative stress of S. obliquus at the concentration of 1 mg/L and had environmental toxicity.