In this work GaN films are grown on 4H-SiC substrates with different surface terrace step width and the influence of the step width on the crystal quality of GaN is studied using X-ray diffraction (XRD) method. The results prove that the different surface terrace step width has a big impact on the (0 0 2) and (1 0 2) full width at half maximum (FWHM) of GaN films. It is mainly because the nucleation and coalescence mechanism of AlN is closely related with the surface step width, resulting in different densities of threading dislocations in the subsequent grown GaN films.