Background: The pathogenic of gram positive Staphylococcus aureous and gram negative E. coli are the most important infectious factors for human. Since the bacterial resistance to the chemical medicines has increased and the plant medicines are of less side effects, nowadays the medical plants have been the focus of attention. Objective: In this research antibacterial effects of aqueous extract of Oxalis corniculata L., (Oxalidaceae) were evaluated on Staphylococcus aureous and E.coli species. Methods: The disc diffusion method was used in this study due to it's simple, short time procedure and reliability. After preparation of aqueous extract, it's antibacterial effects were assessed at 5, 10, 15 and 20 percent (50μL) concentrations. Next, the obtained results were compared with antimicrobial effects of common antibiotics on S. aureous and E. coli. Results: The resullys showed that the average of circle diameters made by Oxalis corniculata L. was more than that made by chemical medicines. Increasing the concentration of the extract from 5 to 20 percent, the average of the circle diameters increased. Conclusions: The final results of the research showed that in all cases there was a direct relationship between concentration and circle of inhibition diameter. In the other word, antibacterial effect increased with the increase of extract concentration.