Objective: To investigate antioxidant activities of some selected plants used to treat diabetic patients by Thai folk doctors.
Material and Method: Ten Thai plants were selected, macerated and extracted with 95% ethanol. DPPH scavenging and lipid peroxidation assays were used to test the antioxidant activities of the extracts.
Results: Mammea siamensis (Saaraphee), Tacca chantrieri (Narapusri) and Albizia myriophylla (Cha Em Thai) showed the highest antioxidant activity on DPPH radical assay (EC50 = 10.17, 10.24 and 14.45 μg/ml, respectively). Mimussops elengi (Pikun), M. siamensis (Saaraphee) and A. myriophylla (Cha Em Thai) exhibited the highest antioxidant activity by lipid peroxidation assay (IC50 0.39, 0.43 and 0.70 μg/ml, respectively). Six of ten plant extracts showed antioxidant activity on both assays (IC50< 20 μg/ml).
Conclusion: These plant extracts containing in Thai folklore antidiabetic medicine, which showed high antioxidant activity, may be used effectively for the treatment of diabetic patients.

Keywords: Antioxidant, DPPH, Lipid peroxidation, Diabetic, Thai medicinal plants