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GaN hexagonal microdisks were fabricated on Ti-mask (4 nm in thickness) nanohole-patterned m-plane (10-10) GaN substrates by radio frequency-plasma-assisted molecular beam epitaxy. The GaN hexagonal microdisks, which were supported by ~300 nm-diameter GaN nanocolumns, consisted of thin hexagonal c-plane plates with a hexagon side length of 1-2 m and a typical thickness of 200 nm. The GaN hexagonal microdisks were optically pumped under a high optical excitation density with a 355-nm-wavelength Nd:YAG laser, and ultraviolet lasing actions on the quasi-whispering gallery mode (WGM) were observed at room temperature. The lasing wavelength was 372 nm and the threshold excitation density was approximately 250 kW/cm2. The quasi-WGM resonance was numerically analyzed using a simple plane wave model and a 2-D-flnite difference time domain method, the behaviors of the WGM and quasi WGM were analyzed, revealing that the quasi-WGM has a higher Q-factor, thus, it was clarified that the laser actions of the GaN hexagonal microdisks occurred on the quasi-WGM resonance.