The chemical structures of eight compounds purified from two plants (Polygala tenuifolia Willdenow and Dioscorea batatas Decene) were determined and their anti-microbial activity against three microbial strains (Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans) was tested. The three micro organisms were cultured in 96-well plates or Petri dishes without (control) or with the eight compounds added at concentrations of 100 to 0.01 ${mu}M$ (wt/vol). The growth of the microorganisms in the medium was examined after a 24-h incubation. The inhibitory effect of each compound on the growth of the microorganisms was calculated from the optical density measured at 595 nm, turbidity, and size of the inhibition zone around the treated paper disc. The minimum inhiitory concentration (MIC) of compounds 4 to 7 against S. aureus was 0.08, 0.05, 1.3 and 0.02 ${mu}M$, respectively, and 0.09, 0.1, 0.2 and 100 ${mu}M$ against C. albicans. The $IC_{50}$ (50% inhibition) values of compounds 5 and 6 were 3.1 and 6.4 ${mu}M$ against S. aureus, respectively, and 10 and 2.4 ${mu}M$ against C. albicans. Therefore, compounds 4 to 6 were the most potent anti-microbial agents among the eight compounds tested.