The Mekong Delta of Vietnam is an alluvial land that is regularly accrete and thus possesses fertile soils that is very acordant for the development of natural medicinal materials. On the other hand, studies on bioactive medicinal plants grown here are limited, especially those with strong antioxidant activity are of interest. The objective of the study is to evaluate preliminary antioxidant activity on extracts of some medicinal herbs such as Mangifera indica L., Psidium guajava L., and Clinacanthus nutans (Burm.f.) Lindau collected from a region in the Mekong Delta area. Antioxidant activity was analyzed with DPPH and ABTS free radical scavenging assay with ascorbic acid as control. The presence of analyzed chemical constituents in the extract such as flavonoids, saponins, alkaloids, steroids, tannins, triterpenoids, coumarins and reducing sugars. Ethanolic and aqueous extracts of Mangifera indica L. and Psidium guajava L. both showed better antioxidant activity than Clinacanthus nutans (Burm.f.) Lindau when investigated on two methods DPPH and ABTS. Thus, this study is the basic premise of potential antioxidant biological activities of medicinal species, thereby aiming at the prospect of exploiting and applying diverse natural medicinal sources in food technology, and biopharmaceuticals.