Monolaurin, pennyroyal (Mentha pulegium L., Labiatae) and spearmint (Mentha spicata L.) essential oils have various antibacterial activities on microorganisms. Enhancement of antibacterial activity and detect their combination effects on B.cereus ATCC 11778 and E. coli O157:H7 were the purpose of this investigation. Monolaurin preparation, spearmint and pennyroyal oils preparation and their chemical composition analysis with GC-MS method, bacterial inoculums preparation, antimicrobial susceptibility testing using broth micro dilution MIC testing and finally statistic analysis of results with SPSS software package were the material and methods used in this research. MIC (minimum inhibitory concentration) of pennyroyal oil, spearmint oil, monolaurin, monolaurin-pennyroyal oil combination and monolaurin-spearmint oil combination on B.cereus and E. coli O157:H7 were significant (p<0.01). The MIC of monolaurin-pennyroyal oil combination and monolaurin-spearmint oil combination on B.cereus in comparison with the MIC of pennyroyal and spearmint oils separately were significant (p<0.01). Monolaurin-spearmint oil combination on E.coli O157:H7 synergistic inhibitory effect was shown. The most effective antimicrobial agents on B.cereus were monolaurin, monolaurin-spearmint oil combination and monolaurin-pennyroyal oil combination, and the least effective agent was spearmint oil. Also the most effective antimicrobial agent on E. coli O157:H7 was pennyroyal oil, and the least effective agent was monolaurin. Since the MIC of monolaurin-pennyroyal oil combination and monolaurin-spearmint oil combination on E.coli O157:H7 in comparison with using them separately were not significant, so for significant effect on E.coli O157:H7 and other gram negative bacteria, combination of monolaurin with chelator agents and other natural antimicrobials is suggested.