||The aim or this study was to assess the antimicrobial properties of 30 essential oils, extracted from four different Sardinian aromatic plants (Rosmarinus, Eucalyptus, Helichrysum, Thymus), and of their major components, in order to evaluate the relation between chemical composition and antimicrobial activity. The chemical composition of the oil samples was determined by gas-chromatography. The antimicrobial properties of the oils and their components were evaluated using a broth microdilution method. The chemical analysis of the essential oil samples showed a high variability among different species and also in the percentage of the main components within the same species. In general, Thymus oils had the highest antimicrobial power while Helichrysum was the least active. Gram positive bacteria and yeasts were more sensitive than Gram negatives. The relation between clinical composition and antimicrobial properties was particularly evident for Thymus oils: in fact, thymol and carvacrol turned out to be the most active among the authentic compounds tested and similarly the three Thymus oils which contained the highest percentages of these phenols also showed the highest antimicrobial activity.