Background: The establishment of various stem cell sources that is effective in treating a variety of diseases and free from both ethical obstacles and obtaining difficulty is extremely crucial in stem cell therapy. Therefore, we tried to examine the potentiality of the human umbilical vein endothelial cells (HUVECs) as a new, appropriate stem cell source for ameliorating progressive heart failure in an acute myocardial infarction (MI) rat model. Methods and Results: In an acute MI rat model, HUVECs, which had been injected directly into the infarct border zone, effectively attenuated the left ventricular systolic dysfunction and remodeling compared with those of the control group (left ventricular end-diastolic dimension (mm); MI-Control (n = 6): 11.23짹0.57 vs. MI-HUVECs (n = 6): 10.04짹0.89*, left ventricular end-systolic dimension (mm); 9.33짹0.45 vs. 8.34 짹0.65* and left ventricular end-diastolic pressure (mmHg); 18.41짹6.08 vs. 8.16짹5.55*, *p value<0.05). In the same tissue, a population of injected HUVECs which expressed both cardiac maker (cTnI) and constituted gap junction (connexin 43) with an adjacent rat cardiomyocytes were observed. However, a lot of HUVECs without differentiation into specific cell type existed. In addition, eNOS expressed cells were detected at infarct area. In vivo zymography analysis, HUVECs decreased the activation of matrix metalloproteinase. In immunohistochemistry, decreased matrix metalloproteinase-2 expression of infarct area was observed at 48 hours after cell injection. These effects were inhibited by L-NAME (NOS inhibitor, 10 mg/kg). In vivo zymography analysis, the activation of tissue inhibitor of metalloproteinase was not changed by HUVECs. Conclusions: Our results showed that HUVECs can be considered a new cell candidate for ischemic heart failure after acute MI via an inactivation of matrix metalloproteinases via eNOS.