The crystal quality improvement of a-plane GaN grown on r-plane sapphire was demonstrated by applying thermal annealing on as-grown samples in nitrogen ambient. The root mean square roughness of the 1000 °C-annealed a-plane GaN was only 0.4 nm measured by atomic force microscopy. Transmission electron microscopy results further indicate threading dislocations were decreased from 5×1010 cm-2 to 1.5×1010 cm-2 along GaN and stacking faults were decreased from 8.7× 105 cm-1 to 4.8× 105 cm-1 after the sample was annealed at 1000 °C. Room temperature photo luminescence measurements showed band edge emission intensity was enhanced up to 2.6 folds compared to the regular a-plane GaN film. Furthermore, corresponding cathode luminescence images reveal larger emission area for a-plane GaN with annealing than those without annealing, which was attributed to reduction of the nonradiative recombination centers. A series of experiments confirm this annealing process could be useful for further applications of GaN-based optoelectric devices.