Objective: To evaluate the in vitro antioxidant power of cactus pear seed oil [. Opuntia ficus-indica L. MILL. (CPSO)] and its protective effect against chemically induced diabetes mellitus in mice. Methods: The in vitro antioxidant effect of CPSO was evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging assay. The preventive effect was conducted on Swiss albino mice treated with CPSO (2 mL/kg, per os), before and after a single intraperitoneal alloxan administration (100 mg/kg). Survival rate, body weight and fasting blood glucose were measured and histopathological analysis of pancreas was performed to evaluate alloxan-induced tissue injuries. Results: CPSO exhibited an antioxidant effect in DPPH scavenging assay. Moreover, the administration of CPSO (2 mL/kg) significantly attenuated alloxan-induced death and hyperglycemia (P < 0.001) in treated mice. Morphometric study of pancreas revealed that CPSO significantly protected islets of langerhans against alloxan induced-tissue alterations. Conclusions: Based on theses results, CPSO can prevente alloxan-induced-diabetes by quenching free radicals produced by alloxan and inhibiting tissue injuries in pancreatic β-cells.