The formation of well-aligned ZnO nanorods has been achieved via H-2 treatment of as-grown ZnO films. Structural analyses reveal that the ZnO nanorods on the ZnO films are preferentially oriented along the c-axis direction and exhibit a single-crystalline wurtzite structure. To investigate the mechanism of formation of ZnO nanorods on the film, further H-2 treatment of the as-grown ZnO nanorods was performed. Thinner and longer ZnO nanorods were obtained after certain periods of H-2 treatment. It is proposed that both etching and re-deposition processes are taking place during the process, resulting in the aspect-ratio enhancement of the ZnO nanorods and the formation of ZnO nanorods on the ZnO films. It is suggested that an appropriate concentration of the etching products remaining from the initial rod-forming H-2 treatment allows subsequent re-deposition of the ZnO nanorods with enhanced differentiation of the growth rates on the <001> and <100> crystal facets.