Diamond was deposited on mirror-like polished, grinded, and grooved 4H-SiC substrates by microwave plasma chemical vapor deposition. The nucleation process and grain morphology varied with different growth time and concentrations of methane in hydrogen were observed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. It was shown that the nucleation density increased by three orders of magnitude on the grinded substrate, compared with mirror-like polished substrate. The concentration of methane is critical factor that affected the nucleation density, grain size and developing facet. Furthermore, the transformation from nucleation to continuous film was revealed on side wall of the grooved substrate. And the nucleation growth mechanism of diamond on the 4H-SiC substrate was discussed, and a four-stage nucleation growth mechanism has been proposed with the formation of carbon clusters by converting from sp1 to sp2 and sp3 carbon, shaping into preferred crystal planes and forming a well-defined cubo-octahedral diamond grains before producing a compact and coalesced diamond film.