Ethanol leaf extracts of the four common plants in the Mekong delta of Vietnam include Annona glabra L., Imperata cylindrica L. (Beauv.), Limonia acidissima L. and Ruellia tuberosa L. were selected to determine the total polyphenol and saponin contents and to test for their antioxidant and inhibitory effects against two selected bacterial plant pathogens. Among the extracts tested, R. tuberosa L. contains the highest amount of total polyphenol compounds (121.42 mg GAE/g extract). The total saponin contents in the four extracts were detected from 18.7 to 48.8 mg/g. Based on their DPPH and hydrogen peroxide radical scavenging activities, the antioxidant effects of R. tuberosa L. extract was found to be the strongest among those of the four extracts. Indeed, its IC50 value in the DPPH test was 37.6 µg/ml and that in the hydrogen peroxide test was 62.55 µg/ml. All four extracts exhibited their inhibitory effects against the two selected bacterial plant pathogens, i.e., Erwinia carotorova and Klebsiella variicola, exccept L. acidissima L. for K. variicola. The diameters of inhibition zones using dual culture tests varied from 2.0 to 10.7 mm for E. carotorova and from 2.3 to 6.8 mm for K­. variicola.