The genus Dorema (Apiaceae) is represented in the flora of Iran with seven species of which two, D. ammoniacum D. Don. and D. aucheri Boiss. are endemic. Ripe fruits of D. ammoniacum collected just in the deciduous time were subjected to hydrodistillation to yield the essential oil, which was subsequently analyzed by GC and GC-MS. Twenty-nine compounds were identified and quantified, representing 95.1 % of the total oil. (Z)-Ocimenone (22.3 %) and (E)-ocimenone (18.1 %) were the main components of the oil. In vitro antimicrobial activity of the oil was evaluated against seven Gram-positive and Gram-negative bacteria (Bacillus subtilis, Enterococcus faecalis, Staphylococcus epidermidis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae) and three fungi (Candida albicans, Saccharomyces cerevisiae and Aspergillus niger). The results of the antimicrobial assay of the oil by the disc diffusion method and the MIC values indicated that the oil exhibited moderate to high antimicrobial activity, especially against B. subtilis and S. epidermidis with MIC value of 3.75 mg ml-1.