Together with sapphire, SiC is the most common substrate material for GaN epitaxial growth. In fact, SiC has advantages over sapphire because of its better thermal conductivity and lower film substrate lattice mismatch (∼3.5%). However, nucleation of GaN on SiC is rather difficult because of the low surface energy of SiC and the sensitivity of substrate preparation. This latter point makes it essential to use a very careful cleaning step, and also to pre-treat the substrate surface by growing a thick buffer layer of AlN at a relatively high temperature. In this study, several pre-treatment steps of SiC for GaN deposition were tested including (a) nitration with NH3 for 0.5-20 minutes, (b) pre-adsorption of trimethyl gallium (TMG) or trimethyl aluminum (TMA) for 0.5-5 minutes, and (c) deposition of an AlN buffer layer at ∼1150°C. After each pre-treatment, GaN was deposited by MOCVD using dilute H2(Ar+12%H2), NH3 and TMG. All the films were characterized by XRD and cross-sectional TEM. After nitration of SiC, the deposited GaN film was found to be polycrystalline. In case of pre-adsorption of TMG, epitaxial but island-like GaN formed on the substrate. In the third case, with an ultra-thin (∼1.5nm) coverage of AlN on SiC (by pre-adsorption of TMA or by 50 seconds deposition of AlN), GaN epilayers were successfully deposited on SiC. However, when AlN was deposited for longer than 3 minutes (up to 10 minutes), only polycrystalline GaN was obtained. With this technique of covering the surface with an ultra-thin layer of AlN, epitaxial GaN has been successfully deposited on 6H-SiC (0001), on 4H-SiC(0001), and on 3C-SiC/Si(111) substrates. The effect of the different pre-treatments of SiC on the quality of the deposited GaN films will be discussed and compared, and the optimal conditions for GaN deposition for each substrate will be presented.