Type 1 diabetes mellitus is a chronic disease characterized by lack of insulin production. Immune mechanisms are implicated in the pathogenesis of Type 1 diabetes. Canarium odontophyllum (CO) fruits and leaves have been shown to possess high antioxidant activity. This study was conducted to evaluate the effects of CO leaves aqueous extract on the blood glucose and T lymphocyte population in the spleen of streptozotocin (STZ)-induced diabetic rats. Nineteen male Sprague-Dawley rats were randomly divided into three groups: normal, diabetic control and CO treated diabetic groups. Diabetes was induced by a single intraperitoneal injection of 65 mg STZ/kg body weight. The extract of CO leaves was administered orally by force feeding daily at the dose of 300 mg/kg for 28 days. The rats were sacrificed at the end of the study and the spleen was harvested for flow cytometry analysis. The results showed a significant decrease in body weight of diabetic and CO treated diabetic groups compared with the normal group (p