The self-assembled ZnO microcrystalline film was synthesized through repeated growth by a low-temperature hydrothermal method using p-GaN wafer as a template. Well-defined peaks with 6-fold symmetry in the XRD -scan indicated the repeated grown ZnO microcrystal following an orientation relationship of [103]ZnO||[103]GaN. Room temperature photoluminescence (RT PL) spectra indicated that self-assembled ZnO owned a strong ultraviolet (UV) emission accompanied by a weak defects-related emission. The electrically pumped single-mode lasing emission located at 407 nm with a full width at half-maximum (fwhm) of 0.7 nm was observed based on the self-assembled n-ZnO microcrystalline film/p-GaN heterojunction diode.