〔Abstract〕 Objective To investigate the partial mechanism and effect of transplanting with human umbilical cord-derived mesenchymal stem cells(hUC-MSCs) to repair articular cartilage defects in a rat model. Methods Critical-sized osteochondral defects were created in the trochlear grooves of rat femurs. The hUC-MSCs were transplanted into the defect of experimental knees. Sham group and model group knees were transplanted with saline. The effects of articular cartilage repair were evaluated at 10 weeks after surgery with International Cartilage Repair Society(ICRS) repair score, histological examination and immunohistochemical analysis.The effects of hUC-MSCs on the proliferation and migration of chondrocytes were detected by Transwellin vitro. Results After transplanting with hUC-MSCs, the articular surfaces of the defect sites were changed smoother thanthose of the model group, and the cellular morphology and arrangement were also improved in Safranin O staining or Masson staining results. Similar to surrounding normal articular cartilage tissue after treatment for 10 weeks, and the ICRS repair score was significantly elevated. In addition, hUC-MSCs treatment could promote chondrocytes proliferation, increase the expression of type Ⅱ(Col Ⅱ) collagen and decrease the level of type Ⅰcollagen(Col Ⅰ) in the articular defect site.Meanwhile, the increased protein expression of SOX9 and protein transportation to the nucleus were observed after treatment with hUC-MSCs for 10 weeks.In vitro, chondrocytes could be proliferated and migrated after co-cultured with hUC-MSCs. Conclusion Transplanting with hUC-MSCs can promote cartilage repair, and its role maybe related to the protection of the cartilage matrix and the promotion of chondrocyte proliferation and migration.