Direct graphene growth on insulating substrates is of great importance for graphene electronics. Limited accomplishments by using molecular beam epitaxy have been demonstrated on substrates of hexagonal symmetry. For comparison and further progress, we study the growth of graphitic carbon on cubic MgO substrates. Raman spectra clearly show D, G, and 2D peaks, confirming the formation of nanocrystalline graphite. The degree of graphitization is comparable to those of carbon layers grown on hexagonal substrates. X-ray photoelectron spectroscopy proves the dominance of carbon sp 2 bonding, and transmission electron microscopy reveals nanometer-scale clusters directly. The flatness and the homogeneity of graphitic carbon on MgO(100) are also beneficial to potential applications of heterostructures containing graphitic carbon. First-principles calculations elucidate that the strong carbon-oxygen interaction limits the in-plane coherence length.