The antioxidant effect of t9, t11-CLA was investigated based on the indicators of the activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and the content of malondialdehyde (MDA) in rats serum and liver tissue. The results showed that after feeding for 4 weeks, the activities of SOD and GSH-Px in the serum and liver tissues of middle and high dose group were distinctly higher than that of control group（P<0.05）, while the change of the content of MDA was opposite, which indicated that t9, t11-CLA had good protective effect on oxidative stress injury in rats and this protective effect came from their own antioxidant capacity as well as the activated antioxidant enzymes of the body.