Six new flavonoids 2',4'-dihydroxychalcone-6'-O-β-d-glucopyranoside (1), α,3,2',4'-tetrahydroxy-4-methoxy-dihydrochalcone-3'-C-β-glucopyranosy-6'-O-β-d-glucopyranoside (2), 7-hydroxy-5,8'-dimethoxy-6'α-l-rhamnopyranosyl-8-(3-phenyl-trans-acryloyl)-1-benzopyran-2-one (3), 6'7-dihydroxy-5,8-dimethoxy-8(3-phenyl-trans-acryloyl)-1-benzopyran-2-one (4), 9-hydroxy-3,8-dimethoxy-4-prenylpterocarpan (5), and α,4,4'-trihydroxydihydrochalcone-2'-O-β-d-glucopyranoside (6) were isolated from bark of Eysenhardtia polystachya. Antidiabetic activity of compounds 1-5 in terms of their cellular antioxidant and free radical scavenging and also in streptozotocin- (STZ-) induced diabetic mice was evaluated on liver transaminases, lipid peroxidation, total bilirubin, total protein, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (CSH-Px), and glutathione reductase (GSH). Results indicated that 1-5 scavenged 2,2-diphenyl-1-picrylhydrazyl (DPPH), hydroxyl (∙OH), nitric oxide radicals (NO∙), superoxide anion radical (O2∙-), radical cation (ABTS∙+), and hydrogen peroxide (H2O2) radical, and protection against H2O2 induced BSA damage was also observed. Furthermore, 1-5 showed ability to decrease the oxidative stress in H9c2 cell. Diabetic mice present high levels of lipid peroxide, total protein, SGPT, SGOT, ALP, and TB. However, treatment of STZ-induced diabetes in mice with 1-5 reduced levels of these enzymes leading to protector effect of liver. In addition, with treatment with 1-5, increases in radical scavenging enzymes of CSH-Px, SOD, GSH, and CAT have also been observed in diabetic mice. The antioxidant properties of compounds 1-5 are a promising strategy for ameliorating therapeutic effects by avoiding disorders in the normal redox reactions in healthy cells which consequently could alleviate complications of diabetes.