Aim To investigate the long-term effects of transplanting mononuclear bone marrow stem cell (MBMC) on repairing necrotic myocardium in chronic heart failure rats. Methods First,to establish chronic heart failure rat animal model. MBMC isolated from the rats were directly injected into the myocardium of transplantation group, control group were injected PBS, and sham-operated rats were underwent the surgery but not transplantation. At 4 th, 8 th and 12 th week after transplantation, myocardium were assessed by immunohistochemistry and electron-microscope. Results In heart specimens slice, by immunofluorescence-microscope, it was confirmed that there were newborn cardiomyocyte and blood vessels at the site of transplantation which nucleolus were marked with fluorescence. Myogenic cell similar with cardiomyocyte,desmin and actin could be detected by immunohistochemistry, proliferating cell nuclear antigen(PCNA) detected the proliferated cell. Immature cardiomyocyte,cell ultrastructure changes and newborn blood vessels which endothelium were not complete could also be detected by electron-microscope. All these matter became more and more with time went on. There were no significant discoveries between control and sham-operation groups, and at different time, there appeared dead cases. Conclusions MBMC transplantation can effectively prevent heart function deteriorated in chronic heart failure, regenerate cardiomyocyte and blood vessels. There are no bad responses such as new ischemia, inflammation and maliganant arrhythmia. MBMC can continue to repair necrotic myocardium. The long-term effect is notable.