Ethanolic extracts of 30 Thai medicinal plants, traditionally used as alternative treatments in diabetes, were evaluated for antioxidative activity by the 2,2-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) method. They were evaluated in vitro for oxidative stress by thiobarbituric acid-reactive substance (TBARS) assay in pooled plasma of diabetic patients compared to without treatment of the extracts (control). The extracts were also assayed for protein glycation. The results showed that five plants had strong antioxidant activity: Phyllanthus emblica Linn. (PE), Terminalia chebula Retz. (TC), Morinda citrifolia Linn. (MC), Kaempferia parviflora Wall. (KP) and Houttuynia cordata Thunb.(HC), respectively. Thirty plant extracts were good correlation between total antioxidant activity and antiradical activity by TBARS as well as by glycation (r = 0.856, p < 0.01 and r = 0.810, p < 0.01). PE had stronger antioxidative activity as well as inhibition of TBARS and glycation than the other plants. The investigation showed that total polyphenol and tannin content of PE and the flavonoid content of HC were the highest. The results imply that these plants are potential sources of natural antioxidants which have free radical scavenging activity and might be used for reducing oxidative stress in diabetes.