Silicon carbide (SiC) is rapidly becoming the substrate of choice for the development of high frequency and high power electronic devices employing the III-V nitride family of materials. This heteroepitaxial growth system continues to receive considerable attention, as materials issues remain the fundamental limiters to device performance. The heteroepitaxial growth of gallium nitride (GaN) thin films on stepped and step-free 4H SiC surfaces is reported. Step-free SiC surfaces are created by mesa patterning of a SiC wafer and subsequent epitaxial growth in a process described previously. This process results in a collection of both step-free and stepped surfaces on a given sample. We have employed an established metalorganic chemical vapor deposition process to grow first a thin (1200Å) aluminum nitride (AlN) nucleation layer and then a 2 μm thick GaN thin film. We have interrupted growth at various stages of AlN and GaN growth to evaluate the growth evolution using atomic force microscopy (AFM). The results show marked differences in the manner in which the initial AlN layer deposits. Nucleation is random with elongated grains on step-free SiC surfaces, while stepped surfaces have round nuclei of uniform dimensions and a high degree of spatial correlation with the nuclei arranged in rows. These differences diminish as the AlN layer approaches the desired thickness. Growth of the GaN epilayer is also markedly different on the two types of surfaces with step-free surfaces leading to random and low density nucleation of crystallites that remain as single grains for long growth times, whereas the stepped surfaces have large numbers nuclei that rapidly grow laterally. Cross-sectional transmission electron microscopy (TEM) reveals that grain sizes are 2–3X larger on step-free surfaces.