Objective To explore the promotive effect and mechanism of human cord blood mononuclear cells (HCMNCs) on angiogenesis in rats with acute myocardial infarction ( AMI). Methods HCMNCs were isolated and labeled with BrdU. Twenty rats were selected to establish the acute myocardial infarction model by coronary artery ligation. The rats were randomly divided into myocardial infarction and HCMNC groups with 10 rats in each group. Another 10 rats were used as the sham operation group. After successful model establishment, HCMNCs were injected around the myocardial infarction in the HCMNC group and L-DMEM was injected into the same site of sham operation and model groups. Four weeks after transplantation, echocardiography was used to assess cardiac functions, HE staining was used to observe myocardial pathological changes, BrdU staining was used to detect transplanted cell survival, and immunohistochemical staining was used to analyze the microvessel density (MVD) and protein expression of CD31 vascular endothelial growth factor (VEGF). Western blott was used to measure CD31 and VEGF protein levels in myocardial tissue. Results Compared with the sham operation group, the left ventricular end systolic dimension ( LVEDs ) and left ventricular end diastolic dimension (LVEDd) were increased and the left ventricular ejection fraction (LVEF) and fraction shortening (FS) were decreased in myocardial infarction and HCMNC groups (P<0. 05). Compared with the myocardial infarction group, LVEDs and LVEDd were decreased, and LVEF and FS were increased in the HCMNC group (P<0. 05). HE staining showed that the structure of myocardial cells was normal in the sham operation group and the arrangement of myocardial cells was disordered in the myocardial infarction group with a large amount of inflammatory cell infiltration. The structure of myocardial cells in the HCMNC group was essentially normal. BrdU-positive cells were not detected in sham operation or myocardial infarction groups and scattered BrdU-positive cells were seen in the infarct area of the HCMNCs group, which were involved in formation of the blood vessel wall. Compared with the sham operation group, relative expression of CD31 and VEGF proteins and MVD were decreased in myocardial infarction and HCMNC groups (P<0. 05), and compared with the myocardial infarction group, relative expression of CD31 and VEGF proteins and MVD were increased (P< 0. 05). Conclusions HCMNCs promote the establishment of collateral circulation in AMI rats, induce angiogenesis, and significantly improve ischemic heart functions.