خلاصه مقاله: Introduction: Today, bacterial infections and antibiotic resistance are major challenges for medical practioners. In this regard, herbal antimicrobial properties due to their availability and limited side effects, have been of interest to the researchers.This study attempts to investigate in vitro the antibactrial effects of aqueous extract of Pistacia atlantica on the isolates of Escherichia coli from urine specimens collected from women with suspected urinary infection tract (UTI). Materials and Methods: The antibacterial effect of the extract was determined by broth microdilution method on 40 clinical isolates of E. coli and standard strain (ATCC 25922). Different concentrations of P. atlantica were prepared 0.2-200 mg/ml in96-well plate wells, respectively. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined by using colony count on blood agar medium. The collected data were statistically analyzed by SPSS (V. 22) software using Chi-square and t-test. Results: The Mean± SD of MIC and MBC of P. atlantica aqueous extract were 46.8±3.8 and 93.7±77.3 on clinical isolates, respectively. The results showed the greatest antibacterial effects in 50 mg/ µl concentration on clinical isolates of E. coli. However, no significant difference between MIC and MBC of ATCC (MIC and MBC were 100 and 200 mg/µl,respectively) was revealed among the clinical isolates (p= 1.0). Conclusion: Based on the results, the aqueous extract of P. atlantica exhibited antibacterial potency against the clinical isolates of E. coli and could be affective as a natural antimicrobial agent.