Background: The objectives of our study is to assess the MRI findings of sacroiliac joint in patients with sacroiliitis related ankylosing spondylitis and to evaluate the active and chronic changes of sacroilitis. Methods: The study entrolled forty patients suffered from low back pain and diagnosed with ankylosing spondylitis. They were referred from Rheumatology Clinic to the MRI unit to be recruited in the study. Each patient who participate in the study obtained counseling and a written informed consent. Results: MRI study plays an important role in the diagnosis of sacroilitis related ankylosing spondylitis MRI can detect the active and chronic changes sacroiliitis. we found different abnormal findings of sacroiliac joint on MRI which assess the disease activity. Conclusion: In this study, we found that MRI study shows the active and chronic changes in sacroiliac joint in patients with ankylosing spondylitis, which CT study and conventional radiography can detect these chronic changes later.