GaN epilayers were grown by MOVPE using a low temperature AlN nucleation layer. The substrate temperatures for the deposition of the nucleation layer and the main layer are about 720°C and 1145°C respectively. Cross-section TEM used to observe the image of GaN epilayer on silicon substrates. These GaN epilayers exhibit a specular surface, suggesting a two-dimensional growth mode. The observed microstructure shows the threading dislocation density is ∼1×1010cm-2. The lattice mismatch is found to be relieved by a periodic array of misfit dislocations.