Objective:To investigate the effect of human umbilical cord mesenchymal stem cells on the proliferation and metastasis of human hepatocellular carcinoma cell line(HepG2)in vitro and in vivo. Methods:The UC－MSCs were cultured 48 hours in serum－free DMEM/F12 medium,and then the supernatant(MSCs－CM)was collected. The MSCs－CM was added to the culture medium of HepG2 cells to a final concentration of 25%,50% or 75%(the experimental groups). DMEM/F12 was set as control group. Cell proliferation was detected by cell counting Kit－8(CCK－8)and the migration and invasion were assessed by Transwell assays. Nude mouse subcutaneous tumor models were established. A total of 12 male nude mice were randomly divided into 3 groups(n = 4,respectively):Group A(subcutaneous injection of HepG2 on the left side),Group B(subcutaneous injection of UC－MSCs + HepG2 on the left side)and Group C(subcutaneous injection with HepG2 on the left side and UC－MSCs+HepG2 on the right side). The volume of tumor was monitored at interval of 7 days until all the mice were sacrificed for the measurements of wet tumor weights at 50th day. Results:After treatment with each concentration of MSCs－CM,the proliferation of HepG2 was significantly promoted(P < 0.05) and the invasiveness was also significantly improved(P < 0.01)compared with the control group,respectively. Significant differences of wet tumor weight and volume were observed between Group A(0.054 2 ± 0.011 2 g)and Group B(0.292 0 ± 0.156 9 g),as well as two sides of Group C(left side:0.089 4 ± 0.024 2 g;right side:0.332 6 ± 0.102 9 g,P < 0.05)mice in a time－dependent manner after 50 days. Conclusion:UC－MSCs can significantly promote proliferation and metastasis capability of HepG2 cells via increasing the release of cytokines of UC－MSCs.