Abstract: Background and Objective: Leaves of basil plant (Ocimum basilicum) are often used for medicinal purposes because of their bioactive constituents, yet the antioxidant properties of this plant are not fully studied in the field of diabetes. The present study investigated the antioxidant property and anti-diabetic effect of two basil cultivars of Ocimum basilicum, ‘Italian Genovese’ and ‘Thyrsiflora’, in a Streptozotocin (STZ) rat model of diabetes . Materials and Methods: Sixty adult Sprague Dawley rats (n = 10/group) were divided into 6 groups: Three non-diabetic and three diabetic groups that either did not receive any supplementation or were supplemented with the leaves extract of one or the other cultivar. After 13 weeks of feeding, all rats were sacrificed, pancreatic tissues were homogenized and used for evaluating oxidative DNA damage and dichlorofluorescein fluorescence (DCF) assay. Blood was collected for the measurements of glucose and insulin. Results: The STZ caused oxidative stress in the diabetic group as evidenced by an increase in oxidative DNA damage and also caused DCF production in pancreatic tissues as compared to non-diabetic groups, (p<0.05). The STZ treatment resulted in hyperglycemia and low serum insulin level in diabetic rats. Supplementation with extracts of ‘Italian Genovese’ and ‘Thyrsiflora’ to the diabetic groups significantly abrogated the STZ-mediated effects (p<0.05). Conclusion: The results indicated that the extracts from the leaves of the two examined basil cultivars act as potent antioxidants and combat the STZ-mediated diabetogenic effect.