Abstract: In this study, we investigated the effect of different concentrations of water-soluble fraction of No.0 diesel oil (DWSF), the most used fuel oil in China, on antioxidant enzymes activities of Manila clam (Ruditapes philippinarum). The activities of catalase (CAT), glutathione S-transferase (GST), superoxide dismutase SOD and peroxidase (POD) were significantly induced or depressed in hepatopancreas and gills of R.philippinarum at different concentrations and time intervals. Generally, the response of antioxidases in gills exhibited higher sensitivity than that in hepatopancreas. Among these four antioxidase, CAT, SOD and POD were highly sensitive to No. 0 diesel oil exposure, especially POD activity in gills with the highest induction of approximate three times of the control. Integrated biomarker response (IBR) index reflected that the antioxidant defense systems of R.philippinarum were more sensitive to the high concentration of DWSF. Thus, it is concluded that the water-soluble fraction of No.0 diesel oil candamage the antioxidant enzyme system of R.philippinarum, demonstrating great potential as biomarkerin biotesting of marine oil pollutions.