Objective　To investigate the effects of bone marrow stem cells (BMSCs) transplantation on heart function of acute myocardial infarction (AMI) in rats and to observe their survival and differentiation in damaged host myocardium.　Methods　AMI was created by occluding the left anterior descending artery (LAD) in SD rats. BMSCs of healthy rats were isolated, purified, amplified and labeled with colloidal gold. The rats were randomly divided into 3 groups according to the means dealing with BMSCs and the components injected into myocardium, with 12 rats for every group. Group A received exogenous transplantation of BMSCs，but group B received exogenous transplantation of BMSCs induced by 5-aza. Group C (control group) was injected 100μl IMDM. At the 4th week after transplantation, index of hemodynamics were tested. Survival and differentiation of the transplanted BMSCs were observed by immunohistochemistry.　Results　(1) The difference in cardiac function among three groups were remarkably significant (P<0.01) at 4th week after intramyocardiac injection. There were no significant difference between group A and B. (2) LVEDP was lower in treatment group than control group (P<0.01), LVSP and ±dp/dtmax were higher in treatment group than control group (P<0.01). Transplanted cells differentiated into cardiomyocytes with positive cTnT and α-sarcomeric actin and survived in area of inferepicardium, myocardial scar and besided normal myocardium.　Conclusions　The exogenous BMSCs can improve the heart function remarkably when transplanted into infarcted myocardium and can differentiate into viable cardiomyocytes.