Herbs have been used as traditional medicine for centuries and believed to possess antioxidant properties. We screen aqueous extracts from 40 medicinal plants in Thai traditional medicine that believed to reduce the risks of cardiovascular diseases, diabetes and cancers. Total antioxidant activity was measured by Trolox equivalent antioxidant capacity (TEAC) assay. In the plants associated with the treatment of cardiovascular diseases, we found that the total antioxidant activities of Nelumbo nucifera Gaertn., Bixaorellane, Michelia alba, Mammea siamensis, and Michelia champaca were higher than others. In the plants associated with the treatment of diabetes, we found that total antioxidant activities of Xylinbaria minutiflora, Leptocarpus disjuntus, Salacia chinensis, Albizia myriophylla and Urceola minutiflora were higher than others. In the plants associated with the treatment of cancers, we found that total antioxidant activities of Balanophora abbreviata, Alpinia allughas, and Datura fastuosa were higher than others. Our results suggest Thai medicinal plants as valuable sources of natural antioxidants.