Mesenchymal stem cells (MSCs) have generated a great deal of interest in clinical application because of their potential use in regenerative medicine and tissue engineering. However, the therapeutic application of MSCs still remain limited unless the favorable effect of MSCs for tumor growth in vivo and the long-term safety of the clinical applications of MSCs are better understood. In this study, MSCs derived from fetal bone marrow (FMSCs) and adult MSCs (AMSCs) alone or FMSCs and AMSCs with tumor cell line (F6 or SW480) together were transplanted subcutaneously into BALB/c-nu/nu mice to observe the outgrowth of tumor, and the characteristics of tumor cells were investigated by pathological and immunohistochemical methods, flow cytometry and real-time quantitative PCR. The results showed that both FMSCs and AMSCs could favor tumor growth in vivo. The pathologic examination revealed that tumor tissues had rich vessel distribution, extensive necrosis and invasion surrounding normal tissues, such as muscular tissue and subcutaneous tissue. In the immunohistochemical examination, tumor cells mixed with MSCs transplanted subcutaneously exhibited elevated capability of proliferation, rich angiogenesis in tumor tissues and highly metastatic ability. To understand whether MSCs affected the general properties of the tumor cells in vivo, the expression of some surface antigens and Bmi-1 gene of tumor tissue cells was detected in this study. The results indicated that these parameters were not affected after the interaction of MSCs with tumor cells in vivo. These findings suggested that MSCs could favor tumor growth in vivo. It is necessary to carry out a study for assurance of the long-term safety before MSCs were used as a therapy tools in regenerative medicine and tissue engineering.