Background: Acinetobacter baumannii is an opportunistic pathogen and a nosocomial infection agent. Antibiotic resistance in this organism results in serious health problem such as treatment failure and increased mortality. Mangrove plants, especially Avicennia marina, are a potential bioresource for new drugs as they have been used in traditional medicine. This study aimed to investigate antibacterial effects of A.marina leaf extract on standard and clinical stains of A.baumannii in vitro. Materials and Methods: A.marina leaves were dried in outdoor shade and ground. Then glycerin extract was provided at a concentration of 200 µg/ml and its antibacterial properties were assessed on standard strain and carbapenem-resistant clinical isolates of A.baumannii using agar well diffusion method. Minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were determined. Results: A. marina leaf extract had a significant antibacterial effect on all 6 strains at a concentration of 100 µL in agar well diffusion method. MICs were 25µg/ml in all strains. MBCs were 12.5µg/ml in standard strains. The most resistant strain against the extract was cefepime-resistant clinical strain. Conclusion: A. marina leaf crude extract had significant antibacterial effects on standard and carbapenem-resistant clinical isolates of A. baumannii.