In this study, a GaN microdisk is fabricated on a sapphire substrate by hydride vapor phase epitaxy (HVPE). The microdisk has a flat surface and a regular hexagonal shape with a diameter and height of approximately 27 μm and 15 μm, respectively. Photoluminescence (PL) experimental results show that the optical resonance modes of the microdisk are different in the vertical and horizontal orientations, with the latter supporting the whispering gallery mode (WGM) oscillation. When the microdisk is irradiated by a high-energy pulsed laser at room temperature with an excitation light power larger than 7.8 μW, a multimode laser signal near 374 nm wavelength is obtained in the PL spectra, in which the WGM laser is dominant, and the quality factor can reach 3742. Finally, using the COMSOL simulation software, a light field simulation is performed on the resonant cavity to analyze its optical mode characteristics.