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We have studied the growth and characterization of patterned and uniformly distributed GaN microcrystals with well-defined facets and epitaxy. The microcrystals were grown on a mask pattertned by lithography, and formed by selective-area epitaxy using metal-organic chemical vapour deposition. The GaN microcrystals have similar sizes and shapes. Each microcrystal consists of an upper and lower part, which are rotated by 30 deg. Transmission electron microscopy shows that there is a rather clear interface between the two parts of the crystal, suggesting a sudden change in the growth direction. We performed ab initio calculations for the surface energies of hexagonal GaN, and the growth morphology is explained based on surface energy consideration.