The intracellular antioxidant activities of diosmetin were evaluated by cellular antioxidant activity (CAA) assay, 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH)-induced erythrocyte hemolysis assay and cupric chloride (CuCl2)-induced plasma oxidation assay. The results showed that diosmetin exhibits strong cellular antioxidant activity (EC50 = 7.98 μmol, CAA value = 58 μmol QE/100 μmol). It was also found that diosmetin treatment could effectively attenuate AAPH-induced erythrocyte hemolysis (91.0% inhibition at 100 μg/mL) and CuCl2-induced plasma oxidation through inhibition of intracellular reactive oxygen species (ROS) generation. Diosmetin could significantly restore AAPH-induced increase of intracelluar antioxidant enzyme (SOD, GPx, and CAT) activities to normal levels, as well as inhibit intracellular malondialdehyde (MDA) formation. Thus, the intracellular antioxidant detoxifying mechanism of diosmetin is associated with both nonenzymatic and enzymatic defense systems.