The differentiation function and paracrine action of mesenchymal stem cells (MSCs) have been reported previously, although it is not clear which accounts for the short-term effects on post-MSC transplantation. In this study, rats with acute myocardial infarction (AMI) induced by occlusion of the left coronary artery were randomized to the MSC transplantation group and MI group. The differentiation and paracrine action of MSC were observed after 2 weeks of MI. We found that MSC transplantation improved cardiac function [left ventricular end-systolic pressure (LVESP), left ventricular end-diastolic pressure (LVEDP), left ventricular +dp/dt and ejection fraction (EF)]. Only a few grafted MSC differentiated into myocardial cells or vascular endothelial cells. Conversely, a significant increase in capillary density was observed. MSC transplantation upregulated the expression of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). These results indicate that an increase in capillary density and improved cardiac function post-MSC transplantation are closely related to the up-regulation of VEGF and bFGF.