In this work, the major purpose was to study the surface treatments of GaN substrates by chamical mechanical polish(CMP) and thermal anneal and expect to obtain epi-ready GaN substrate. Both photoluminescence and cathodoluminescence indicate that CMP and thermal anneal both effectively removed the polishing-induced damage layer. However, thermal anneal result in an unacceptably rough surface morphology and the higher the anneal temperature, the rougher surface become. The longtime CMP polished surface shows low roughness with no scratches and produced a root-mean -square (RMS) surface roughness less than 1.5nm. Finally we acquire the optimal CMP process parameter for GaN substrates and it was successful to grow high quality u-GaN by MOCVD on GaN substrates with no damage layer. We can evidence the quality of u-GaN by AFM、Nomarski、XRD and CL measurements.