The methanol extract of 12 medicinal plants were evaluated for its antibacterial activity against Gram-positive (5 strains) and Gram-negative bacteria (10 strains) by assay for minimum inhibitory concentration (MIC) and minimum bacterial concentration (MBC). The antibacterial activity was determined by an agar dilution method (according to the guidelines of Clinical and Laboratory Standard Institute). All the compounds (12 extracts) of the 8 medicinal plants (leaf or root) were active against both Gram-negative and Gram-positive bacteria. Gram-negative showed a more potent action than Gram positive bacteria. The MIC concentrations were various ranged from $0.6;{mu}g/ml$ to $5000;{mu}g/ml$. The lowest MIC ($0.6;{mu}g/ml$) and MBC ($1.22;{mu}g/ml$) values were obtained with extract on 4 and 3 of the 15 microorganisms tested, respectively.