Herbs have been used for medicinal purposes for centuries and known to possess antioxidant properties that may help to reduce the risk of chronic diseases, cardiovascular disease, and cancer. We screen aqueous extracts from 20 medicinal plants in Thailand that were believed to possess anti-tumor activity, help immune-stimulating property and maintain blood stasis. The antioxidant activities were investigated in two bioassays. Firstly, we demonstrated inhibition of Heinz bodies induction caused by oxidants under in vitro condition. The percentages of Heinz body inhibition activity in plant extracts from Terminalia citrina, Cassia timoriensis, and Derris elliptica were the highest followed by Anamirta cocculus, and Oroxylum indicum respectively. In addition, we investigated total antioxidant activity in plant extracts by improved ABTS radical cation decolorization assay. The total antioxidant activity of the extract from Terminalia citrina was also the highest activity followed by Ficus pubigera, Derris elliptica, Anamirta cocculus, Caesalpinia sappan, and Oroxylum indicum respectively. Our results suggest medicinal Thai plants as valuable sources of antioxidants, which may have a potential anti-carcinogenic activity.