Objective:To explore the mechanism of treadmill exercise on knee articular cartilage of mice with high fat diet. MethodsThirty male C57BL-6 mice were equally divided into the normal group,high fat diet group and treadmill exercise group. The mice in the high fat diet group and treadmill exercise group were fed with 40% high fat diet for 8 weeks. Then the treadmill exercise group was treated with moderate treadmill training,the treatment course was 45 minutes per time,5 times every week and total 4 weeks. The morphological changes of knee articular cartilage in mice were observed by HE staining and Safranine and Fast Green double dyeing. The expression levels of matrix metalloproteinase-13(MMP-13)and low-density lipoprotein receptor related protein 5(LRP5)in knee joint were determined by immunohistochemistry,and the mRNA expression levels of MMP-13,LRP5 and β-catenin in knee articular cartilage were detected by RT-PCR.  Results:Compared with the high fat diet group,the body weight and Lee’s index of mice in the treadmill exercise group were decreased(P<0.05). The Mankin score suggested that there was obvious degeneration in knee articular cartilage in the treadmill exercise group(P<0.05). The mRNA expression of MMP-13 in articular cartilage was increased in the treadmill exercise group compared with the high fat diet group(P<0.05). The immunohistochemical results also showed that the expressions of MMP-13 and LRP5 in knee joint were increased in the treadmill exercise group compared with the high fat diet group(P<0.001).  Conclusion:Moderate treadmill exercise can aggravate the knee osteoarthritis process in high fat fed mice,and the increasing expressions of MMP-13 and LRP5 in articular cartilage may induce the knee osteoarthritis.