We have investigated the growth characteristics of thick GaN on Sim substrate with AlN and low temperature GaN buffer layer. The vertical hydride vapor phase epitaxy system with GaCl3 GaCl3 precursor was used for growth of GaN. AlN and GaN buffer layer were deposited on Si substrate to reduce the lattice mismatch and the thermal expansion coefficient mismatch between si and GaN. Optimization of deposition condition for AlN and low temperature GaN buffer layers were carried out. We studied the effects of growth temperature, V/III ratio on the properties of thick GaN. Surface morphology, growth rate and crystallinity of thick GaN were measured using Atomic Force Microscopy (AFM), α-step, Scanning Electron Microscopy (SEM) and X-Ray Diffractometer(XRD).