Background: Ankylosing spondylitis (AS) is a chronic progressive inflammatory rheumatic disease that leads to significant decrease in the quality of life. Mean platelet volume (MPV) is a part of the complete blood count (CBC) test and correlates with the platelets function and activation. Objective: To evaluate the diagnostic value of MPV in patients with AS and to assess its relationships with disease activity index. Patients and Methods: A total of 100 patients with AS (78 males: 22 females) were diagnosed according to the modified New York classification criteria for AS and 146 (99male: 47 female) healthy individuals were matched in age and sex as controls. Demographic data, disease activity scores using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), medical history, C-reactive protein (CRP), erythrocytes sedimentation rate (ESR), and MPV were measured. Results: Mean age of patients was 38.0 ± 9.0 years and controls were 35.8 ± 8.3 years. Males were 78% in patients and 67.8% in controls. The mean BMI was 28.3 ± 5.5 kg/m2 in patients and 28.6 ± 3.8 kg/m2 in controls. There were no statistically significant differences in age, sex, and BMI between patients and controls (p>0.05). MPV was significantly higher in patients with AS compared to healthy controls (9.215 ± 1.57 vs. 7.753 ± 0.86). MPV had a good ability to differentiate between active AS and inactive AS patients. Optimum cut off point was less than 9.9. MPV had the highest accuracy (79%) with high specificity 86.8 %, positive predictive value (PPV) at pretest 50% was 84.9%, and at pretest, 90% was 98.1% with sensitivity 74.2% and negative predictive value (NPV) at pretest 10% was 96.8%. MPV had a significant positive correlation with disease activity. Increased MPV will increase disease activity. Conclusion: MPV was significantly higher in AS patients than in controls and directly correlated with ESR, CRP, and BASDAI.