Abstract: We aimed to compare the antimicrobial effects of essential oils (EOs; Mentha × piperita, M. haplocalyx and M. × gentilis) alone and in combination with conventional antimicrobials against Staphylococcus aureus, Bacillus cereus, Escherichia coli, 2 strains of Pseudomonas aeruginosa and Klebsiella pneumoniae. The major components of EOs extracted from the leaves of M. × piperita, M. haplocalyx and M. × gentilis during flowering were analyzed by gas chromatography/ mass spectrometry. The antimicrobial activity of EOs in single use and combined with antibiotics at a particular ratio (1:1) were tested by the paper-disk diffusion method against the above microorganisms. When used alone, P. aeruginosa ATCC 15442 was sensitive to EOs of M. × piperita and M. haplocalyx and P. aeruginosa ATCC 27853 to EOs of M. haplocalyx and M. × gentilis. When combined with the reference antibiotics, most of the EOs showed larger inhibition diameters and broader spectrum than that used alone. P. aeruginosa ATCC 15442 and P. aeruginosa ATCC 27853 were more sensitive to the combination of the antibiotics with EOs. The combination of M. × piperita EOs and ceftazidime had the strongest synergistic effect against P. aeruginosa ATCC 15442. Only the combination M. haplocalyx EOs with ceftazidime was antagonistic against P. aeruginosa ATCC 27853. The most resistant strain was S. aureus ATCC 25923, which was resistant to all combinations. The combinations of EOs and antibiotics at the ratio of 1:1 exerted their synergistic or antagonistic effects against different microorganisms, which indicated that the differences in major compounds and contents affected the antibacterial efficacy significantly.