This paper reported a simple and effective method for fabricating and patterning highly ordered ZnO nanorod arrays on H2-decomposed GaN epilayer via hydrothermal route. The edge of pattern, which has been decomposed by H2 flow, provides appropriate nucleation sites for the selective-growth of aligned ZnO nanorods. The density of ZnO nanorod arrays assembled the hexagonal pattern can be tuned by varying the solution concentrations, growth time and reaction temperatures. The results have demonstrated that the ZnO nanorods are highly uniform in diameter and height with perfect alignment and are epitaxially grown along [0001] direction. This work provides a novel and accessible route to prepare oriented and aligned ZnO nanorod arrays pattern. And the aligned ZnO nanorods form an ideal hexagonal pattern that might be used in many potential applications of ZnO nanomaterials.