Abstract: In order to assess the effects of cadmium on the antioxidant defense system of fish species, the activities of superoxide dismutase (SOD), catalase (CAT) and content of glutathione (GSH) in muscle, liver, kidney, spleen and gill tissues of Orechromis niloticus exposed to cadmium at concentrations of 0、0.5、1.0、3.0、6.0mg/L for 21 days by static experiment test. The results showed that SOD, CAT activity and GSH content in liver tissues were significantly higher than other tissues (P<0.05). The activity of SOD and CAT, and content of GSH were decreased with cadmium concentration accumulating in liver. There was significantly lower of activities of SOD, CAT and content of GSH in 3.0mg/L and 6.0mg/L for exposure group than those of in the control group (P<0.05). At the same time, SOD and CAT activities and cadmium concentrations showed a strong relationship of stoichiometry with the range of exposure dosages, different induction of cadium to activities of SOD, CAT and content of GSH in liver of Orechromis niloticus juveniles. However, enzyme activities is induced at the concentration of 0.5mg/L, which decreased in gill with the increasing concentration of cadmium (P<0.05). There was no significant difference of SOD and CAT activity between exposure group and the control group in muscle and spleen tissues (P>0.05).