The interface properties and energy band alignment of SiO₂/GaN metal-oxide-semiconductor (MOS) structures fabricated on N-polar GaN([Formula: see text]) substrates were investigated by electrical measurements and synchrotron-radiation x-ray photoelectron spectroscopy. They were then compared with those of SiO₂/GaN MOS structures on Ga-polar GaN(0001). Although the SiO₂/GaN([Formula: see text]) structure was found to be more thermally unstable than that on the GaN(0001) substrate, excellent electrical properties were obtained for the SiO₂/GaN([Formula: see text]) structure by optimizing conditions for post-deposition annealing. However, the conduction band offset for SiO₂/GaN([Formula: see text]) was smaller than that for SiO₂/GaN(0001), leading to increased gate leakage current. Therefore, caution is needed when using N-polar GaN([Formula: see text]) substrates for MOS device fabrication.