Objective To investigate the effects of transplantation with different amount of bone marrow mesenchymal stem cells(BMSCs)on osteoporosis in ovariectomized rats.Methods The rat model of osteoporosis was prepared by ovariectom(OVX)and verified after 3 months.A total of 60 female Sprague-Dawley rats were randomly divided into 6 groups(n=10 each).The rats with shamed operation served as a sham-injured control group(sham control group).The 5 ovariectom (OVX) groups with osteoporosis were study groups as follows:(1)the negative therapy group(simple OVX group);(2)the positive therapy group(OVX+ estrogen,E2 group) by intramuscular injection;others were treated with transplantation of BMSCs by tail vein injection in low dose(LS group),middle dose (MS group)and high dose(HS group).At 8,12 and 16 weeks after intervention,body mineral density (BMD)were detected by dual-energy x-ray absorptiometry scans.After 16 weeks of intervention,rat shinbone was obtained and stained by hematoxylin-eosin(HE) staining.Results Compared with the sham control group,simple OVX group showed a reduced total body BMD and the decreased proportion of trabecular bone to bone marrow cavity area (P ＜0.05).The total body BMD and the proportion of trabecular bone to bone marrow cavity area were higher in each BMSCs transplantation groups than in simple OVX group at 8,12,16 weeks after intervention(P ＜0.05),which showed a increased trend in the total body BMD and the proportion with the increased dose of transplantation BMSCs(P＜0.05).Rats in the HS group had highest BMD and best proportion of trabecular bone to bone marrow cavity area among three doses of transplantation BMSCs.Conclusions BMSCs transplantation can significantly improve osteoporosis of ovariectomized rats with an increased total body BMD and higher proportion of trabecular bone to bone marrow cavity area,and better and longer outcomes can be achieved.