Background: Avicennia marina (Forssk.) Vierh. belongs to Acantaceae family. Due to biological active compounds and traditional use of it’s leaves to treat rheumatic disease and wounds, seems that this plant has significant anti-microbial effects. The aim of this study was to determine antimicrobial effect of the aqueous and ethanolic extracts of Avicennia marina (different concentrations) on Staphylococcus epidermidis PTCC 1435, Streptococcus pyogenes PTCC 1447 and Pseudomonas aeruginosa PTCC 1310” in vitro”. Material &Methods: In this study, antimicrobial effect of the extracts was evaluated by two methods, “Collins method” (spreading of the extract on medium surface) and “disk agar diffusion method”. The Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) for both species determined using a dilution method. Statistical analysis was carried out by analysis of variance (ANOVA). Results: The results showed that in "disk agar diffusion test ", ethanolic extract had inhibitory effect on Staphylococcus epidermidis and Streptococcus pyogenes. The results showed that in, “Collins method” aqueous extract did not show any inhibitory effect on Pseudomonas aeruginosa. The result showed that MIC of Avicenna marina leaves of the aqueous and ethanolic extracts for Pseudomonas aeruginosa were 32 and 16 mg/ml, respectively. The MBC aqueous and ethanolic extracts of Avicenna marina leaves for Pseudomonas aeruginosa were, 64 and 32 mg/ml respectively. Conclusions: Pseudomonas aeruginosa was resistant to most of the aqueous and ethanolic Avicenna marina extracts. The ethanolic extract of Avicenna marina leaves “in vitro” have a significant antimicrobial effect on gram-negative bacteria Pseudomonas aeruginosa and the gram-positive bacteria Staphylococcus epidermidis and Streptococcus pyogenes.