Abstract: Introduction: We previously reported that ankylosing spondylitis (AS) was associated with Behcet's disease (BD) in Iranian patients. This study was designed to evaluate the features of AS in BD (ASBD) and compare them with the AS in normal population (ASN). Materials and Methods: Among 4700 BD patients, 73 ASBD were found and were compared with a cohort of 180 ASN. Seventy out of 73 ASBD met the European Spondyloarthropathy Study Group (ESSG) criteria and 44 the modified New York criteria (mNYC). From 180 ASN, 145 met the ESSG criteria and 100 patients the mNYC. Results: The prevalence of ASBD was 1.55% and the ASN 0,18%. The difference was significant (p<0.0001). In ASBD the mean age at the onset of joint involvement was 26.4 years versus (vs) 27.3 years (p=0.5) in ASN. Males were 67% vs 80% (p=0.03). Constitutional signs were 2.7% vs 24% (p<0. 0001). Morning Stiffness (MS) was 42.5% vs 66% (p<0.0001). Inflammatory low back pain was 57.5% vs 67% (p=1). Peripheral joint involvement was 55% vs 27.2% (p<0.0001). Limitation of lumbar spine motion was 41% vs 62% (p=0.002). Dorsal and cervical spine involvement in ASN was higher (p<0.0001). In ASBD, B27 was positive in 44.5% vs 46.5% (p=0.8) in ASN. Elevated ESR, positive CRP, and radiographic changes were significantly more frequent in ASN. Conclusion: Our study demonstrates: 1-AS is more frequent in BD patients than in the normal population. 2-In ASN, males, constitutional signs, MS, limitation of lumbar spine motion, dorsal and cervical spine involvement, abnormal Schober test, elevated ESR, positive CRP, and radiographic changes were more frequent.