Abstract： Objective To study the repair function of united endothelial progenitor cells (EPC)transplantation on injured liver endothelium by bone marrow transplantation (BMT) conditioning.Methods C57BL/6 mice were divided into four groups randomly: normal control group, without any treatment; irradiation alone group, administered a total body irradiation(TBI) pretreatment, without BMT; (3) BMT alone group: C57BL/6 mice were infused with bone marrow mononuclearcells (MNC) 5 × 106/only through caudal vein not more than 4 h after the same TBI pretreatment as the irradiation alone group; united transplantation group: receiving the same way as the BMT alone group, but C57BL/6 mice were infused with EPC 5 × 105/only at the same time. Two, 4, 7, 14, and 21 days after the TBI, the changes of the liver weight were observed regularly. The histopathological examination of liver was done at the 4th, 7th, 14th, and 21st day after the TBI. Results In irradiation alone group, BMT alone group and united transplantation group the liver weight began to increase significantly on the day 2 and peaked at 14th day after the TBI, and the peaks were respectively (1.65±0. 15) times (P＜0. 05), (1.61 ±0.06) times (P＜0.05), and (1.11 ±0.40)times (P＜0. 05) of those in normal control group. At the day 14, the liver weight in irradiation alone group, BMT alone group and united transplantation group began to decrease, and on the day 21 the liver weight in united transplantation group had been completely restored to normal level, however the liver weight in irradiation alone group and BMT alone group were still significantly heavier than that in normal control group (P＜0. 05). Liver histopathological examination revealed that there were obvious sinusoidal endothelial cells (SEC) injury, hepatocyte edema and severe inflammatory cell infiltration in irradiation alone group, and on the day 7 the hepatocyte edema and necrosis were significantly worse than before, and almost no alive SEC were found. On the day 14 the injury of SEC in BMT alone group was lighter than before, but on the day 21 the injury had not returned to normal. On the day 7 the injury of SEC, hepatocyte edema and necrosis were alleviated in united transplantation group as compared with irradiation alone group and BMT alone group, and on the day 14 the injury had returned to normal basically. Conclusion The transplantation conditioning could damage recipient liver endothelium and the injury would persist, and united EPC infusion could repair the injured SEC following BMT.