Monolaurin, pennyroyal (Mentha pulegium L., Labiatae) and spearmint (Mentha spicata L.) essential oils have various antibacterial activities on microorganisms. Enhancement of antibacterial activity and detection of their combined effects on B.cereus ATCC 11778 and E. coli O157:H7 were the purpose of this investigation. Monolaurin preparation, spearmint and pennyroyal essential oils preparation and analysis of their chemical composition 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