Neuregulin-1β (NRG-1β)/ErbB signaling plays crucial roles in the cardiac differentiation of mouse embryonic stem cells (ESCs), but its roles and the underlying mechanisms in cardiac differentiation are incompletely understood. This study showed that NRG-1β significantly increased the percentage of beating embryoid bodies (EBs) and up-regulated the gene expressions of Nkx2.5, GATA4, α-actin, MLC-2v, and ANF in a time-dependent manner, with no effect on the gene expressions of HCN4 and Tbx3. Inhibition of ErbB receptors with AG1478 significantly decreased the percentage of beating EBs; down-regulated the gene expressions of Nkx2.5, GATA4, MLC-2v, ANF, and α-actin; and concomitantly up-regulated the gene expressions of HCN4 and Tbx3 in a time-dependent manner. Moreover, the up-regulation of transcripts for Nkx2.5 and GATA4 by NRG-1β was blocked by the extracellular signal-related kinases (ERK) 1/2 inhibitor, U0126. However, U0126 could not inhibit the transcript up-regulations of MLC-2v and ANF by NRG-1β. The protein quantitation results were consistent with those of gene quantitation. Our results suggest that NRG-1β/ErbB signaling plays critical roles in the cardiac differentiation of mouse ESCs and in the subtype specification of cardiomyocytes in a time-dependent manner. The ERK1/2 pathway may be involved in the early cardiogenesis, but not in the subtype specification of cardiomyocytes.