Abstract: Inguinal hernia is one of the most common surgical problems in general population and herniorraphy is one of the most common surgical procedures done in general surgery. In recent years, many progresses have been achieved in this field as using prosthetic materials to reduce the recurrence. Although, it has some disadvantages such as adhesions and infections. The goal of this study is to evaluate the effect of prophylactic antibiotic in reduction of post operative infection in these patients. This is an interventional double blind randomized trial on 212 patients undergone Lichtenstein Inguinal Herniorraphy. A total of 100 cases received prophylactic antibiotic and 112 patients did not receive any antibiotic. They were all examined by another surgeon on 3, 7, 14, 30 and 60th day post operative and the results were put in an information sheet. We used SPSS software to analyze the data and Chi-square (χ²) and exact fisher test were used to evaluate the results. Mean age of patients was 51.1�4.6. Range was 20-81 years; 91.5% were male, 8.5% were female. About 5.2% had Diabetes Mellitus (DM), 7.1% had HTN and 6.6% had previous IHD. About 95.5% had simple inguinal hernia, 2.4% had femoral hernia, 2.3% had incarcerated inguinal hernia, 53.3% right side, 42% left side and 4.7% were bilateral hernia. The incidence of infection in antibiotic group was 2% and in group without prophylactic AB was 2.7% which was not stastically significant (p = 0.4). As there is no significant difference between two groups, prophylactic AB has no effect on post operative infection thus, according to high cost of routine antibiotic prescription and possible complications such as resistance and side effects and reactions of these drugs, it is justified to limit the antibiotic to therapeutic purposes and those who have risk factors for severe infections.