Introduction: The origin of 16% to 30% of low back pain is sacroiliac joint. It is difficult to distinguish the sacroiliac joint pain from other types of low back pain using history and physical examination. Radiological imaging has little role in diagnosis of the sacroiliac joint pain and diagnostic blocks are the gold standard in these patients. The main objective of this study is to evaluate the accuracy and safety of diagnostic sacroiliac joint block (DSJB) under sonographic guidance. Materials and Methods: This analytic cross sectional study was conducted on 65 patients. DSJB under sonographic guidance was done for all patients with 1 ml radiographic contrast material and 1ml local anesthetic (2% lidocaine). In order to confirm intra-articular injection by sonographic guidance, fluoroscopic spot images were considered. The side effect and positive predictive value of physical provocative tests of sacroiliac joint were evaluated. Results: Of 65 diagnostic blocks, after fluoroscopic confirmation, 62 injections were intra-articular and only 3 were peri-articular. There was a significant statistical correlation between procedure time and patients' body mass index. 3 and 4 to 6 positive provocative tests of sacroiliac joint resulted in a positive predictive value of 83.3% and 93.3%, respectively. The procedure had no side effect and complication. Conclusion: Based on the obtained results, diagnostic block of sacroiliac joint under sonographic guidance has reasonable accuracy without side effects. If performed by experienced clinicians in nerve block under sonographic guidance, it can be valuable alternative method for other guidance modalities in diagnosis of patient with sacroiliac joint pain.