Abstract: Objective To study the correlation between haptoblobin (Hp) level in synovial fluid (SF) and severity of osteoarthritis(OA) and the mechanism of Hp underlying the pathogenesis of OA. Methods Forty-one arthroscopy-confirmed knee OA patients and 18 non-OA patients (16 with meniscus injury and 2 with discoid meniscus but without significant cartilage injury) admitted to our hospital from July 2011 to February 2012 were included in this study. Their SF was collected. The OA patients were classified according to the Outerbridge cartilage scoring scale and their HSS knee scores were recorded. Hp levels in SF were measured by ELISA and compared in OA patients at different severity. Results The Hp level in SF was significantly higher in OA patients than in non-OA patients (23.04±13.19 μg/ml vs 7.32±3.76 μg/ml, P＜ 0.001) and negatively correlated with the knee HSS score in OA patients (r=-0.768, P＜ 0.001). Conclusion Hp level in SF is positively correlated with the severity of OA. Measurement of Hp level in SF is of great importance for the early diagnosis and effective prevention and treatment of OA.