Objective To investigate the effect ofestrogen on osteoarthritis in female rats.Methods Forty female rats were divided into four groups. In group Ⅰ, the rats were not given any treatment as a control. Ingroups Ⅱ, Ⅲ and Ⅳ, the rats received fixing left knee joint on extension position. Meanwhile, therats received ovariectomy in group Ⅲ; ovariectomy and diethylstilbestrol treatment in group Ⅳ, respectively. After 4 weeks, histological observation and serum BGP examination were done.Results In groups Ⅱ, Ⅲ andⅣ, the levels of serum BGP were 3.50±0.39, 5.72±0.64 and 3.95±0.44, respectively. The pathologic grades of cartilage and synovium were 10.83±4.35 and 4.21±2.03; 15.32±3.42 and 7.62±3.42; and 12.65±2.73 and 5.46±1.23, respectively. Conclusion Estrogen may play an important role in delaying the development of osteoarthritis.