Deltamethrin (DM), one of the regular pyrethroid pesticides, have toxical effects on aquatic animals after entering the natural water due to human activities. As a largesized crustacean species, crayfish (Procambarus clarkii) is an important link of eco-system of the water. In order to understand the toxical effects and mechanism of DM on P.clarkii, an acute semistatic toxic test was carried out, and the oxidative stress relative indicators (activities of superoxide dismutase (SOD), catalase (CAT) and concentrations of malondialdehyde (MDA) in hepatopancreas) were examined at intervals (0,6,12 and 24 h) after treated with DM (with the concentration of 0.01,0.02 and 0.04 μg/L) by spectrophotometry. LC₅₀ values of DM for P.clarkii in 24, 48 and 96 h were 0. 156 0、0.099 3 and 0.056 2 μg/L, respectively; and its safe concentration was 5.62 ng/L. In all groups treated with DM, the oxidative stress relative indicators varied. The activities of SOD and CAT showed the similar variation during the treatments: inhibitioninductioninhibition , while the levels of MDA were higher in all treated groups than in the control group. After 6 h treated with DM, MDA level in the group of 0.01 μg/L was significantly higher than in the control group（P<0.01），1.98 times higher in the group of 0.04 μg/L than in the control group（P<0.05）; After 12 h treated with DM, MDA level was 1.76 times higher in the group of 0.02 μg/L than in the control group（P<0.05）.The activities of CAT of three treated group were 70.98%、73.05% and 66.67% respectively higher than the control group after 24 h DM treated（P<0.01）; After 48 h treated with DM, SOD activities decreased by 60.38% and 45.60% in the group of 0.01,0.02 μg/L, respectively（P<0.01）.The three different treated groups did not show apparent dosedependent effects. The results provide evidences that DM performed an extremely high toxical effect on P.clarkii and may play its adverse impact through an oxidative damage pathway within 48 h 〖WTBX〗P. clarkii〖WTB1〗 could be used as a bioindicator to the pyrethroid pesticides pollution in the water as it is highly sensitive to pyrethroids.