More than 500 clinical trials are using mesenchymal stem cells (MSCs) in the world to treat some different diseases. The safety of expanded MSC transplantation is the most important thing to ensure that this therapy can become the routine treatment for human diseases. More than five MSCs based stem cell drug products are approved at various countries demonstrated that expanded MSCs are safe in both local injection and transfusion. Moreover, some recent reports for 5 and 10 years followed-up clinical trials using expanded MSCs confirmed that there is not different tumorigenesis between the patients with and without expanded MSC transplantation. This letter aims to provide some evidences about the safety of expanded MSCs in clinical applications. However, the MSC quality should be stritcly controlled during the in vitro MSC expansion.