Stem cell-based therapy in cardiac tissue engineering is an emerging field that shows great potential for treating heart diseases. However, even preliminary issues, such as the ideal niche for cardiomyocytes, have not been clarified yet. In the present study, the effects of extracellular matrix (ECM) components on the beating duration of neonatal rat cardiomyocytes (RCMs) and on the cardiac differentiation of P19.CL6 carcinoma stem cells were studied. RCMs were cultured on gelatin-, fibronectin-, and collagen type I-coated dishes and on noncoated polystyrene dishes, and their beating rate, beating duration, and cardiac gene expression were evaluated. The beating period and the expression of troponin T type-2 (TNNT2) and troponin C type-1 (TNNC1) of cardiomyocytes cultured on gelatin-coated dishes were longer and higher than for those on dishes with other coatings. For the cardiac differentiation of P19.CL6 cells, troponin T type-2 expression on gelatin- and fibronectin-coated dishes was five times that on collagen type I-coated dishes or polystyrene dishes 11 days after induction. These results indicate that a gelatin-coated surface has a high ability not only to maintain the cardiac phenotype but also to enhance cardiac differentiation.