The synergistic antimicrobial activities of blended essential oil preparations composed of eucalyptus, rosemary, patchouli, pine and cajuput oils were evaluated against various pathogenic microorganisms. They exhibited antimicrobial activity in the agar disc diffusion assay against the Gram-positive bacteria, Staphylococcus aureus and S. epidenidis, the Gram-negative bacteria, Escherichia coli and Pseudomonas aeruginosa, and the yeast, Candida albicans. The minimum inhibitory concentration (MIC) of these preparations was evaluated by the broth microdilution method. We found that the best synergistic antibacterial preparation (the sum of fractional inhibitory concentrations, Σfic < 1) was eucalyptus, rosemary, and mineral oils (volume ratio 4:4:2) without patchouli, pine or cajuput oils added, while most preparations showed an antagonistic anticandidal effect (Σfic > 1). The blended essential oil preparations were characterized for their components by GC/MS, and contained 1,8-cineole as a major component. However, the preparation containing the highest amount of 1,8-cineole did not exhibit the best synergistic effect, which was shown by the preparation comprising the highest amount of α-pinene. Hence, we concluded that the differential antibacterial effect of either blended oil preparations or single/pure essential oils was influenced by the amount of α-pinene and the number of active components in either the blended preparations or single/pure essential oils.