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 microg/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 microg/ml, respectively). Six often plant extracts showed antioxidant activity on both assays (IC50 < 20 microg/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.