Abstract: A laboratory experiment was carried out to study the 96 h LC50 of nonylphenol (NP) to Paphia undulate. Acute toxic experiment showed the 96 h LC50 of NP to Paphia undulate was 0.26 mg/L. The changing tendency of superoxide dismutase (SOD)activity, peroxidase (POD) activity, glutathione (GSH)content and malondialdehyde (MDA) content which exist in P. undulata's mantle were simultaneously observed when P. undulate was exposed to low, middle and high concentration of NP (1, 10 and 25 g/L) and after the stress was released. The results show that: after Stress Day one, SOD activity of the mantle of P. undulate was slightly inhibited only at low concentration, then showed a trend of first and then decreased. The activity of POD was only inhibited at the time of Day 15 after exposing to low, medium concentration NP group, but each concentration group were almost induced at the other time. The content of GSH on Day one and Day seven were basically lower than the control group, but on Day 15 the content of GSH was significantly higher than that of control group at three different concentrations(P 0.01).The content of MDA was increased significantly with the prolonging stress time. After the release with clear water, SOD activity and POD activity, GSH content returned to normal level in the low concentration group (1 g/L); only the content of GSH had a better restoring in the middle and high concentration groups (10, 25 g/L). This study shows that NP has marked oxidative damage to P. undulate's mantle, and the damage increased with the increasing concentration of NP. SOD activity, POD activity of the mantle, MDA content and GSH content basically cannot recover to the control level of high concentration of NP stress released after the experiment.