The adult mammalian heart has limited capacity for regeneration following injury, but the neonatal mouse heart possesses remarkable regeneration capacity within 7 days after birth. Defining the molecular mechanisms that govern regenerative capacity in the neonatal period remains a central goal in cardiac biology. A common principle observed in tissue regeneration is the reactivation of the previously employed developmental transcriptional programs. The mechanisms in heart development also govern cardiac regeneration of the injured heart. Therefore, understanding the transcriptional program regulation during heart development is of critical to guide cardiac regeneration therapies. Inducible pluripotent stem cell (iPSC) derived cardiomyocyte has been developed for disease research and drug development, and these cells are now gradually entering the clinical research phase for the testing of heart regeneration therapies. However, a major hurdle for their applications is the immature state of these cardiomyocytes.Cardiac development is orchestrated by multiple layers of epigenetic regulation in a collaborative and strictly temporal manner. New high-throughput sequencing approaches have begun to reveal a new layer of epigenetic regulation, mRNA methylation (N6-methyladenosine, m6A). The m6A is installed by the methyltransferase complex consisting of methyltransferase-like 3 (Mettl3) and Mettl14. The recent discovery of the broad role of m6A mRNA methylation in regulating cell fates, embryonic and brain development, and stress response in adult brain highlights the importance of another layer of epigenetic regulation at the RNA level. Conditional Mettl14 knockout in nerves system resulted in abnormal brain development and early death after birth. Delayed differentiation/maturation due to prolonged cell cycle or impaired neuron proliferation are two possible underlying mechanisms. The role of m6a mRNA methylation in cardiac development is still largely unknown. Cardiac-specific Mettl3 knockout mice showed no evident heart development deficiency. However, we discovered that cardiac-specific Mettl14 KO mice (cMettl14 KO) displayed severe cardiomyopathy resulting death within two months after birth. The mechanisms of the cardiomyopathy in cMettl14 KO mice are unclear. We performed m6A sequencing on the hearts from WT and cMettl14 KO mice at the age of 5 days. We found that many of the decreased m6A tagged transcripts in cMettl14 KO mice were related to cardiomyocyte differentiation, proliferation, maturation or hypertrophy. Among them, the m6A level and mRNA level of Bone Morphogenic protein 2 (BMP2), BMP10 and Wingless-Type MMTV Integration Site Family, Member 5A (WNT5a) were significantly decreased in cMettle14 KO mice. Importantly, we found that the proliferation of cardiomyocyte was increased in cMettl14 KO mice, while the maturation of the cardiomyocyte, assessed by calcium transient, mitochondrial function and the level of fetal/adult myofilament iso...