The implantation of bone marrow mesenchymal stem cells (BMSCs) into the heart has been reported to be effective for myocardial infarction; however, it is unknown what methods are most suitable for supporting stem cell growth in a myocardial patch. This study used a new polymer material composed of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] co-cultured with BMSCs to create a myocardial patch. Bone marrow mesenchymal stem cells were obtained from healthy male BSL-C57 mice. The cells were treated with 5-azacytidine to investigate their differentiation into cardiomyocytes. The cells were seeded for 24-hours onto P(3HB-co-4HB) biological material films (n=8). Cell-biomaterial constructs were fixed and analyzed using different methods. Bone marrow mesenchymal stem cells were CD34, CD45(-), CD90(+) (low), and CD73(+) . The cells were stained with anti-cardiac troponin T (anti-cTnT) and anti-connexin 43 (anti-CX43) antibodies after 5-azacytidine treatment. Scanning electron microscopy showed that BMSC morphology was normal, and cell numbers were more abundant on the P(3HB-co-4HB) material surfaces. The growth curve of BMSCs on the biomaterial patches showed that the P(3HB-co-4HB) material permitted good stem cell growth. Owing to its excellent biocompatibility and biodegradability properties, in particular its porosity, P(3HB-co-4HB) is highlighted as an optimal material to support myocardial cell growth and myocardial patch formation in patients with myocardial infarction.