Gallium oxide (Ga2O3) has been successfully grown on top of an n-type gallium nitride (GaN) substrate via thermal oxidation process which is performed in nitrous oxide ambient at 1000 °C. Effects of oxidation times on physical properties of the thermal oxide have been investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). XRD analysis discloses the presence of monoclinic β-Ga2O3 which is the most thermodynamically stable form of Ga2O3 on the GaN substrate after thermal oxidation. FTIR spectra shows a Ga-O vibration mode at the band around 620-660 cm−1 for all the oxidized samples. Besides, FESEM and AFM results indicate that protrusions of grains are revealed on the surface after thermal oxidation. In addition, surface roughness of the oxide was also found to be increased with the increasing oxidation duration.