We report first-principles electronic structure calculations that clarify the electronic structures of mismatch-free SiC hetero-crystalline superlattices consisting of 3m bilayers of the 3C polytype and 2n bilayers of the 2H polytype, (3C)m(2H)n. We find that electrons in the conduction-band bottom are confined in the 3C region and are distributed near one of the two inequivalent interfaces. This is due to the floating nature of the conduction-band bottom of SiC and electric polarization caused by the difference in electron affinity between Si and C. This polarization affects the conduction-band states and causes the band-gap narrowing of up to 1.2 eV, depending on the thickness of each layer. Our results for the wave-vector-resolved local density of states and the Kohn–Sham orbitals have unequivocally clarified that the valence-band offset is absent. This reflects the fact that the local tetrahedral structures are common in all the SiC polytypes. The amount of the polarization changes with the thickness ...