Chemical mechanical polishing (CMP) of bulk GaN substrates via dry thermal oxidation is investigated in this paper. In this work, we study the effects of oxidation with respect to different thermal treatments, change in morphology and thickness on bulk GaN substrates. The results of the study show that a defect-free surface with roughness average (Ra) and material removal rate (MRR) of 0.377 nm and 51 μm/h respectively is achievable by CMP after thermal treatment at 800°C. However, for thermal treatments above 900°C, several pits and defects are observed with significant deformation of the surface likely due to the domination of diffusioncontrolled reaction over interfacial reaction-controlled. The molar fractions of the chemical components remained on the polished GaN surfaces are characterized via X-ray photoelectron spectroscopy. It is found that the conversion rate from GaN to Ga2O3 is dependent on the real contact area between CMP pad and GaN substrate surface during the CMP.