Abstract: Objective To observe the changes in morphology, structure, and ventricular function of infarct heart after bone marrow mononuclear cells (BMMNC) implantation. Methods Twenty-four dogs were divided into four groups with random number table, acute myocardial infarction (AM I) control group , AM I-BMMNC group , old myocardial infarct ion (OMI) control group and OM I-BMMNC group , 6 dogs each group. Autologous BMMNC were injected into infarct and peri-infarct myocardium fo r transplantation in AM I-BMMNC group and OM I-BMMNC group. The same volume of no-cells phosphate buffered solution (PBS) was injected into the myocardium in AM Icontrol group and OM I-control group. Before and at six weeks of cell t ransplantation, ult rasonic cardiography (UCG) were performed to observe the change of heart morphology and function, then the heart was harvested for morphological and histological study. Results U CG showed that left ventricular end diastolic dimension (LV EDD) , left ventricular end diastolic volume (LVEDV ) , the thickness of left ventricular postwall (LVPW ) in AM I-BMMNC group were significantly less than those in AM I-control group (32. 5±5. 1mm vs. 36. 6±3. 4mm , 46. 7±12. 1m l vs. 57. 5±10. 1m l, 6. 2±0. 6mm vs. 6. 9±0. 9mm; P lt; 0. 05). LVEDD, LVEDV , LVPW in OM I-BMMNC group were significantly less than those in OM I-control group (32. 8±4. 2 mm vs. 36. 8±4. 4mm , 48. 2±12. 9m l vs. 60.6±16.5m l, 7. 0±0. 4mm vs. 7. 3±0. 5mm; P lt; 0. 05). The value of eject fraction (EF) in OM I-BMMNC group were significantly higher than that in OM I-control group (53. 3% ±10. 3% vs. 44. 7%±10. 1% ). Compared with their control group in morphological measurement, the increase of infarct region thickness (7. 0 ± 1. 9mm vs. 5. 0 ±2.0mm , 6.0±0. 6mm vs. 4. 0±0. 5mm; P lt; 0. 05) and the reduction of infarct region length (25. 5±5. 2mm vs. 32. 1±612mm , 33. 6±5. 5mm vs. 39. 0±3. 2mm , P lt; 0. 05) were observed after transplantation in AM I-BMMNC group and OM I-BMMNC group, no ventricular aneurysm was found in AM I-BMMNC group, and the ratio between long axis and minor axis circumference of left ventricle increased in OM I-BMMNC group (0. 581±0. 013 vs. 0. 566±0.015; P lt; 0. 05). Both in AM I-BMMNC group and OM I-BMMNC group, fluorescence expressed in transplantation region was observed, the morphology of most nuclei with fluorescencew as irregular, and the differentiated cardiocyte with fluorescence was not found in myocardium after transplantation. The histological examination showed more neovascularization after transp lantation both in AMI and in OM I, and significant lymphocyte infiltration in AM I-BMMNC group. Conclusion BMMNC implantation into infarct myocardium both in AMI and OMI have a beneficial effect, which can attenuate deleterious ventricular remodeling in morphology and st ructure, and improve neovascularization in histology, and improve the heart function.