Prolonged immunodeficiency after allogeneic bone marrow transplantation (BMT) causes significant morbidity and mortality from infection, while graft versus host disease (GVHD) after allogeneic BMT and immunosuppression therapies against GVHD further deteriorate this process. Adult bone marrow mesenchymal stem cells (MSC) have recently been shown to inhibit T-cell proliferation and reduce GVHD after allo-BMT. In this study, we characterized the effect of MSC on immunoreconstitution and hematopoietic-reconstitution after bone marrow transplantation. BMT model from Fischer344 rats (RT-1Al) to WF rats (RT-1Au) was established for this experiment. Effects of MSCs on hematopoietic reconstitution, immunoreconstitution and GVHD were studied by survival rate, peripheral blood counts, histological analysis and FACS at day +30 after transplantation. Immune function recovery were assessed by lymphocyte proliferation reaction stimulated with ConA and LPS and allogeneic mixed-lymphocyte reaction. At day 30 postransplant, compared with BMT groups, we observed that cotransplantation of MSC and bone marrow promoted the recovery of peripheral blood white blood cells (5.47±1.11x109/L vs7.12±1.70x109/L, p<0.05), lymphocytes and platelets. Accordingly, it was noticed that cotransplantation of MSC with BM not only improved recovery of bone marrow cellularity, but also enhanced B lymphopoiesis (4.66±1.03x109/L vs 6.05±1.39x109/L, p<0.05) and megakaryocytopoiesis (402.50±63.70 x109/L vs 594.33±121.09x109/L, p<0.05). MSC was also shown to improve thymic and spenic architecture reconstitution. Histological analysis showed that near normal thymus architecture and normal spenic architecture, with well-developed red and white pulp and intact lymphoid follicles in the cotransplantation group, while loss of demarcation between cortex and medulla in the thymus and lymphocytic depletion of spenic arteriolar sheaths in BM transplantation rats.The total number of thymus cells and spleen cells were also increased in cotransplantation group compared to only BM transplantation group. Most notable, the higher percentage of CD3+CD4+ was observed in the spleens of cotransplantation group (11.47±3.68% vs 19.14±4.03%, respectively), while no difference in the percentage of CD3+CD8+ between two groups (10.61±3.37% vs 12.13±2.27%, respectively). So the ratio of CD4+/CD8+ cells increased and inclined to normal in cotransplantation groups compared to only BM transplantation group.There are no difference in the percentage of natural killer (NK) cells and monocytes between BMT group and BMT with MSC group. The immuno-inhibitory effect of MSC on reducing GVHD in recipients of an MHC-mismatched BMT was also evident in our study, nonetheless, splenic cells from recipients of MSC cotransplantation group displayed improved non-specific proliferation capability (against ConA LPS stimulation) and alloreactivity (against the third party splenocytes) compared to BM transplantation group. In conclusion, rat bone marrow mesenchymal stem cells cotransplanted with BM improves hematopietic reconstitution and immunoreconstitution with alleviating graft-versus-host disease.