Abstract： Objective Donor derived naive T cells initiated GVHD by contacting with,mesenchymal stem cells(MSC)have been used to prevent or treat graft-versus-host disease(GVHD).although we stiff puzzle about its mechanisms.Observe the effect of MSC on phenotypes of Naive T cell to study the mechanism of MSC immunomodulation. Methods After 3 passages, MSC Was incubated with Naive T cell differentiated from COrd blood CD+34 cells in vitro.Then the variances of naive T cell phenotypes were analyzed by flow cytometric.Results CD+8 T cells were relatively increased after 7 days co-culture with allogenic MSC when compared to control:(35.9±6.3)%VS(18.4±4.5)%.CD+8 CD+3 cells also showed the same trend (27.6±2.8)%vs(15.2±3.1)%.Conclusion MSC may partly reduce the incidence of GVHD by increase of CD+8 naive T cell.The result may provide new clue to explain immunoregulatory mechanism of MSC.