Objective Investigating transplanted time of periosteal mesenchymal stem cell (PMSC) Methods Rat PMSCs were allografted into rat tibia defects made in different time of granulation Then the quantity of formatted bone,i.e.,trabecular volumm rate (TVR),in the defects were measured in histology and bone histomorphometry by 2,3,4 and 8 weeks respectively after transplantation The long bone draft fracture defects of patients were allografted with PMSC of youth children after 2 week of fracture Results TVR was higher and the defects healed more fast in those rats transplanted PMSCs within 2 weeks after the defect were made However there was insignificant difference between the groups of PMSC transplantation was carried out after two weeks of the defect The fractured defects were healed in 15 cases (57 7%) and were not healed in 11 cases (42 3) after 9 months of transplantation Conclusion That rat bone defects were transplanted PMSC within 2 weeks showed expected bone formed effect and insignificant effect on accelerating defect healing by PMSC transplantation in rats or patients after 2 weeks.