The blood-retinal barrier (BRB) breakdown is a common feature of diabetic retinopathy. In this present study, effects of long-term supplementation of alpha-mangostin (α-MG), a xanthone isolated from mangosteen fruit, on the ocular blood flow and blood-retinal barrier were investigated in type 2 diabetic rats. Type 2 diabetes was induced in male Sprague-Dawley rats by feeding with high-fat (HF) diet for two weeks followed by intravenous injection of low dose streptozotocin (STZ; 35 mg/kg body weight). The rats were divided into three groups: control group (CON; n=7), type 2 diabetic (HF-STZ; n=7) group and type 2 diabetic rats supplemented with α-MG (HF-STZ-MG; n=7). The use of animals was approved by Faculty of Medicine, Srinakharinwirot University Animal Ethic Committee. Alpha-mangostin was prepared by dissolving in corn oil in the volume of 1 mL per rat. The daily gavage feeding of α-MG 200 mg/kg body weight/day was performed for 40 weeks. At the start of experiment, the rat was anesthetized with pentobarbital sodium (60 mg/kg BW, i.p.) and a tracheotomy was performed to allow for mechanical ventilation with both room air and supplemented oxygen. A catheter was inserted into the femoral vein to inject Evans blue (EB) dye and a femoral artery was cannulated for measurement of arterial blood pressure. After cannulation was performed, ocular blood flow (OBF) was monitored using Laser Doppler flowmeter. The blood-retinal barrier (BRB) leakage was quantified using Evans blue (EB) dye technique. The effect of α-MG on glycemia was assessed by evaluating glycated hemoglobin (HbA1c). The blood sample for HbA1c assessment was obtained from femoral vein. Values were mean ± S.E.M., compared by ANOVA. In type 2 diabetic rats, HbA1c and MAP were increased significantly, whereas OBF was decreased markedly as compared with control group (p < 0.01). All of these abnormal parameters were improved by α-MG supplementation (HbA1c = 7.31±0.16 mg/dL for HF-STZ vs 4.31±3.33 mg/dL for HF-STZ-MG, p < 0.01; MAP = 132±2.1 mmHg for HF-STZ vs 107.28±3.33 mmHg for HF-STZ-MG, p < 0.01; OBF (arbitary unit) = 173.67±5.0 for HF-STZ vs 230.50±1.68 for HF-STZ-MG, p < 0.01). Additionally, the leakage of EB dye from retinas was significantly decreased in type 2 diabetes supplemented with α-MG as compared with type 2 diabetes (BRB leakage = 47.84±4.67 µg/mg of retina for HF-STZ vs 30.56±1.75 µg/mg of retina for HF-STZ-MG, p < 0.01). These results demonstrated that alpha-mangostin supplementation improved glycemic state, mean arterial pressure and exerted beneficial effects on the ocular blood flow and blood-retinal barrier integrity in type 2 diabetes.