Abstract: A semistatic experiment was conducted to study the effects of different concentrations (low, 32 μg·L^-1; medium, 160 μg·L^-1; and high, 800 μg·L^-1) decabromodiphenyl ether (BDE-209) on the SOD, CAT and POD activities and GSH and MDA contents of Ruditapes philippinarum mantle membrane. Under the effects of BDE-209, the SOD, CAT, and POD activities decreased after an initial increase. After 2 days exposure to low concentration BDE-209, the SOD activity reached the maximum (56.61 U·mg^-1 prot), being significantly higher than the control. No significant difference was observed in the SOD activity between the control and the exposure groups after 7 days recovery in distilled water. After exposure to different concentrations BDE-209 for 2-15 days, the CAT activity was significantly higher than the control, being the highest on the 7th day when exposed to medium and high concentrations BDE-209. The POD activity reached its maximum on the 1st day exposed to medium concentration BDE-209, being 1.33 times of the control, while the POD activity exposed to high concentration BDE-209 was constantly significantly lower till the end of the experiment. With the increasing time of BDE-209 exposure, the MDA content increased after an initial decrease, and had less difference with the control after 3 days recovery in distilled water. The variation trend of GSH was contrary to that of MDA. The GSH content increased significantly at the beginning of the experiment, but decreased with time elapsed, being the lowest at the 3rd day of exposure (P<0.05). The results of this study indicated that BDE-209 could induce the oxidative stress response of R. philippinarum, the CAT, SOD, and POD played an important role in removing oxygen free radical, and the GSH could be regard as a sensitive index for BDE-209 stress.