Mesenchymal stem cells (MSC) are thought to be multi-potent cells that have the potential to differentiate into lineages of mesenchymal tissues, including bone, cartilage, tendon, fat, muscle, and marrow stroma during embryo morphogenesis. In recent years, cells that have the characteristics of mesenchymal stem cells were isolated from marrow aspirates of human and a few animals. It was found that these cells retain the characteristics of stem cells in vitro and could be induced to differentiate exclusively into the osteocytic, chondrocytic, myoblastic and adipocytic lineages. It was demonstrated that MSC could heal clinically significant bone and cartilage defects in animal models. The role of MSC in repairing tendon defect was also testified. In addition, for its multi-potential to differentiate into lineages of mesenchymal tissues, MSC could be used as gene vehicle for gene therapy of trauma care.