The purpose of this study was to investigate whether an artificial matrix can help neonatal cardiomyocytes restore an injured heart in a rat model of myocardial infarction (MI). The left coronary arteries of female Sprague Dawley (SD) rats were ligated to create MI models. Ventricular cardiomyocytes from 1- to 3-day-old SD rats (both sexes) were isolated, cultured, and labeled. Three weeks after MI, the animals were randomized into four groups: (i) group cell plus matrix (n = 12); (ii) group cell (n = 12); (iii) group matrix (n = 12); and (iv) group control (n = 11). Four weeks after transplantation, echocardiography and the Langerdoff model were used to assess heart function. Immunohistochemical staining and polymerase chain reaction (PCR) were performed to track the implanted cardiomyocytes and detect the sex-determining region Y gene on the Y chromosome. Histology study and PCR showed that transplanted cardiomyocytes survived, formed condensed tissue, and produced connected protein in group cell plus matrix. Heart function assessment indicated transplantation of cardiomyocytes plus matrix preserved left ventricle wall thickness, fraction shortening, and end-systolic internal diameter most effectively.