Abstract: Objective To investigate the effects of intravenous administration of Human Umbilical Cord Blood Cells (HUCBC) on the angiogenesis in rabbits with acute myocardial infarction. Methods Rabbits were randomly divided into sham operation group, model and cell transplantation groups, with 15 animals in each group. Rabbits were performed anterior descending branch braid without ligation in sham group; Rabbits acute myocardial infarction models were prepared in cells transplantation group and model group. At hour 24 after model preparation, 0.5 mL HUCBCs suspension (containing 2×107 cells) was intravenously administrated into rabbits in the cell transplantation group. The same volume of saline was administrated into the other two groups. Cardiac functions were performed by echocardiography at 1st week, 2nd week and 4th week after transplantation. The survival of transplanted cells and microvessle density in the myocardium were identified by immunohistochemistry. The expression of VEGF mRNA in the myocardium was examined by RT-PCR. Results Transplantation of HUCBC improved left ventricular function indexes such as LVFS, LVEF at all 3 examinations(P＜0.05); Immunohistochemistry showed that BrdU positive cells were found only in the transplantated rabbits. Compared with control group, there was a significant increase of microvessle density within the boundary of infarcted myocardium in cell transplantation rabbits(P＜0.01). Compared with control group, there was an markedly higher level of the expression of VEGF mRNA in the transplantated group(P＜0.01). Conclusions Angiogenesis induced by the HUCBCs might be the one of main mechanisms for stem cell transplantation for myocardial infarction.