The aim of this study is to compare caudal and lumbar approach in epidural steroids injection for treatment of acute radicular Low back pain. In a randomized clinical trial, 32 patients with radicular low back pain, aged 20-50 years, and confirmed L5-S1 disk herniation that indicated for non surgical treatment were enrolled and assigned into lumbar or caudal group. Drugs are the same in both group consisting triamsinolon 40 mg, lidocaine 50 mg, and normal saline 10 ml, total volume of 13 ml. In lumbar group, the patients received single injection at L5-S1 space in sitting position but in caudal group, they received an injection at sacral hiatus in lateral position. 30 minute after injection, maximum leg raising was measured and pain intensity was measured 30 min, 1 week, and 1 month after the injection based on the visual analogue scale. Patient’s satisfaction and any possible complication such as headache were recorded.