Abstract: The aim of the present study was the evaluation of the antibacterial activity of Fennel essential oil on isolates of Acinetobacter baumannii. Forty eight isolates were collected from clinical specimens from burn wards of hospitals in Tehran, Iran between April and September, 2006. The susceptibility of isolates was determined using a broth microdilution method. Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of isolates to Fennel essential oil were determined. The susceptibilities of isolates to different antibiotics were tested using agar disk diffusion method. The rates of resistance were determined to antibiotics as follows: cefazolin 100%, ciprofloxacin 100%, ofloxacin 95.8%, kanamycin 95.8%, carbenicillin 93.7%, ticarcillin 93.7%, piperacillin 88.9%, co-trimoxazole 79.1%, ceftizoxime 75%, gentamicin 70.8%, cefalotin 60.4%, amikacin 52% and imipenem 14.6%. Fennel essential oil possessed antibacterial effect against all isolates of A. baumannii. These results suggest the potential use of the Fennel essential oil for the control of multi-drug resistant A. baumannii infections. However, more adequate studies must be carried out to verify the possibility of using it for fighting bacterial infections in human.