The crown-like zinc oxide (ZnO) samples, which are composed of a hexagonal cap and a tower-like shaft, are prepared by vapor transport method. The hexagonal cap, working as a whispering gallery mode (WGM) resonant cavity, demonstrates density-dependent ultraviolet (UV) lasing emission with a broadened and squared photoluminescence (PL) profile under UV excitation at 355 nm. Theoretical analyses based on Fermi golden rule show that the broadened spectrum profile results from the special optical mode density characteristics in a WGM micro-cavity, which is in agreement with the observed results.