Objective: To investigate the expression and significance of MMP-3 in synovium of knee joint at different stage in osteoarthritis (OA) patients. Methods: Knee synovial tissue were collected in 90 OA patients (the OA group). Patients in the OA group was divided into 3 subgroups: gradeⅠsubgroup (n=30), grade Ⅱsubgroup (n=30), grade Ⅲ subgroup (n=30). Thirty patients served as control group. Immunohistochemical assay was used to detect the expression of MMP-3 protein in the knee synovial tissue. Results: MMP-3 protein was detected in all knee synovial tissue. The expression of MMP-3 protein in the OA group was significantly higher that in the normal synovium (P<0.05), and the MMP-3 protein was mainly located in the cytoplasm. There was significant difference in the expression of MMP-3 protein between the grade Ⅲ subgroup and the gradeⅠ, grade Ⅱsubgroups (all P<0.05). The expression of MMP-3 protein was positively related to the severity of OA (r=0.912, P<0.05). Conclusions: The expression of MMP-3 protein are closely related to pathogenic mechanism of OA. It may be an important indicator of early diagnosis and the activity of the disease of osteoarthritis.