The trap states at ultraviolet/ozone (UV/O3)-treated Al2O3/GaN interfaces of p-type metal-oxide-semiconductor capacitors (pMOSCAPs) are analyzed through a frequency-dispersion capacitance–voltage (C–V) measurements. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy are applied to confirm a formation of ultrathin oxide layer (Ga2Ox) on GaN surface by the UV/O3 treatment. The trapped charge density and interface trap density improved from 7.30 × 1011 to 2.79 × 1011 cm−2 eV−1 averaged over the bandgap of GaN and from 1.28 × 1013 to 4.08 × 1012 cm−2 eV−1 near the conduction band edge of GaN, respectively, owing to the passivation of Ga2Ox layer at the Al2O3/GaN interfaces. Mechanism for the improved trap states in pMOSCAPs is identified based on the reduced defect states at both Al2O3/Ga2Ox and Ga2Ox/GaN interfaces.