Objective:To determine patts of antimicrobial resistance, analyze the prevalence of oxacillinase and molecular typing of strains of Acinetobacter baumannii (A. baumannii).Methods: A total of 121 strains of A. baumannii were obtained from patients admitted to Imam Hossein and Imam Khomeini Hospitals, Tehran, Iran, from January 2016 to November 2018. Antimicrobial susceptibility testing was performed by Kirby-Bauer disc diffusion method according to the Clinical and Laboratory Standards Institute recommendations. The presence of oxacillinase genes was assessed by polymerase chain reaction (PCR). To determine clonal relatedness, all isolates were subjected to repetitive sequence-based PCR (REP-PCR). Results: The isolates were obtained from 56 (46.3％) males and 65 (53.7％) females with the mean age of 39.5 years. Colistin with 100.0％ sensitivity rate had the highest effect, while ceftriaxone with 16.5％ sensitivity rate had the least effect on A. baumannii isolates. In addition, 96 (79.3％) and 99 (81.8％) isolates were resistant to imipenem and meropenem, respectively. A total of 109 isolates (90.0％) exhibited multiple drug resistance with 10 different resistotypes. In total, 75 (75.7％) of carbapenem resistant isolates were positive for blaOXA-23-like, and 14 (14.1％) for blaOXA-24-like gene. The five main clones A, B, C, D, and E were detected in 25 (25.2％), 36 (36.4％), 10 (10.1％), 8 (8.0％), and 6 (6.1％) of isolates, respectively. Conclusions: Carbapenem-resistant A. baumannii strains are high in the current study. To control the spread of carbapenem-resistant A. baumannii strains, regular monitoring programs are needed.