In this study,indoors static simulation test was used to investigate the effect of different exposure levels (0.005,0.01, 0.025,0.05 and 0.1 mg/L) of phenanthrene on the growth,free radical,antioxidant defenses and malondialdehyde in Scenedesmus obliquus.Results showed that after 96-h exposure at 0.01 mg/L,the growth of algal cells were significantly inhibited and the signal of free radical (g=2.0033,linewidth is 10.15 mT) increased.The superoxide dismutase,catalase,glutathione-s-transferase activities increased significantly after phenanthrene exposure,demonstrating inducibility mechanisms under stress.The glutathione and malondialdehyde content were significantly elevated after exposure to 0.025 mg/L phenanthrene.In conclusion,S.obliquus is sensitive to phenanthrene,since phenanthrene exposure at low level (even 0.01 mg/L) can induce stress effect.The production of free radical may be an important mechanism involved in the toxicities of phenanthrene on S.obliquus,among which the algae growth is more sensitive to damage index.