Objective To examine osteopontin (OPN) levels in both synovial fluid and articular cartilage of patients with primary knee osteoarthritis (OA) and to investigate their relationship with severity of the disease. Methods Fifty patients with knee OA and 10 healthy controls were enrolled in this study.There were 15 males and 35 females with an average age of 61.8±7.4 years in OA group. The control group included 4 males and 6 females with an average age of 63.2±6.0 years. Mankin score were taken to determine the disease severity of the affected knee. The radiographic grading of OA in the knee was performed using the Kellgren-Lawrence criteria. OPN levels in synovial fluid were measured using enzyme-linked immunosorbent assay. OPN levels in articular cartilage were assessed by immunohistochemical methods. Results Compared to healthy controls, the knee OA patients had higher OPN concentration in synovial fluid ([4519.60±1830.37] pg/ml vs. [1179.70±303.39] pg/ml) and articular cartilage([0.60±0.06] vs. [0.43 ±0.07]). In addition, synovial fluid OPN levels showed a positive correlation with articular cartilage OPN levels (r=0.411,P=0.003). Subsequent analysis showed that the OPN levels in synovial fluid significantly had been correlated with severity of disease using Kellgren-Lawrence criteria (r=0.581, P＜ 0.001). Furthermore,the levels of OPN in the articular cartilage also were correlated with disease severity using Mankin score (r=0.675, P＜ 0.001).Conclusion The data suggest that OPN in synovial fluid and articular cartilage is related to progressive joint damage and could be a predictive biomarker respect to disease severity and progression in knee OA.