Objective: Plants offer a novel source for the isolation of a wide variety of medicinal agents. Allium cepa commonly known as onion is very well known medicinal plants and we investigated the antibacterial activity of different extracts and their phytochemical analysis by gas chromatography mass spectrometry (GCMS).Methods: The extracts of A. cepa prepared in six different solvents was analyzed for antibacterial activity against nine American type cell culture (ATCC) reference bacterial strains i.e. Shigella flexneri, Enterococcus faecalis, Staphylococus aureus, Proteus mirabilis, Salmonella typhi, Serratia marcescens, Klebsiella pneumonia, Escherichia coli, Pseudomonas aeruginosa by using the agar well diffusion method. GCMS analysis also has been carried out for their phytochemical analysis.Results: The results obtained from agar well diffusion assay showed the zone of inhibition range from 10±0.76 to 26±0.76 mm for different extracts. The methanol extract was found most potent against K. pneumonia and S. marcescens with the zone of inhibition of 26±0.76 mm for both strains. Minimum inhibitory concentration (MIC) values were in the range of 1.87 to 7.5 mg/ml and the MIC values for K. pneumonia and S. marcescens was 1.87 mg/ml. A total of 43 compounds were identified by GCMS analysis. Out of them dodecanoic acid was found common in all extracts.Conclusion: It is concluded that Allium cepa have good antibacterial activity so it can be used for the treatment of various infectious diseases
