Antimicrobial activities of two blended essential oil preparations comprising lavender oil, petigrain oil, clary sage oil, ylang ylang oil and jasmine oil were evaluated against various pathogenic microorganisms. Both preparations showed antimicrobial activity in the agar disc diffusion assay against the Gram-positive bacteria, Staphylococcus aureus ATCC6538 and S. epidermidis isolated strain, the fungus, Candida albicans ATCC10231, and the Gram-negative bacterium, Escherichia coli ATCC25922, but showed no activity against Pseudomonas aeruginosa ATCC9027. The minimum inhibitory concentration (MIC) of these preparations was evaluated. By the broth microdilution assay, preparation 1, comprising lavender oil, clary sage oil, and ylang ylang oil (volume ratio 3:4:3), exhibited stronger antimicrobial activity than preparation 2, which was composed of petigrain oil, clary sage oil, and jasmine oil (volume ratio 3:4:3). Moreover, the sum of the fractional inhibitory concentrations (Sigma fic) of preparation 1 expressed a synergistic antimicrobial effect against the tested microorganisms (Sigma fic<l). The blended essential oil preparations, characterized for their components by GC/MS, contained linalyl acetate, and linalool as major components. Our experiments showed that the differential antimicrobial effect of either blended oil preparations or single/pure essential oils may be influenced by the amount of linalool and linalyl acetate, and the number of active components in either the blended preparations or single/pure essential oils. In addition, blended oil preparations expressed synergistic antimicrobial effect by the accumulation of active components such as linalool and linalyl acetate and combining active constituents of more than one oil.