We report on the effects of post-annealing on ZnO nanorods array using optical second harmonic generation (SHG). The high-quality and nearly vertically aligned ZnO nanorod arrays were deposited on ZnO-seeded glass substrates by aqueous solution method. The SHG result indicated the transformation of oxygen deficiency on ZnO nanorods surface after post-annealing treatment process. The chemical composition of the nanorods was investigated by X-ray photoelectron spectroscopy. The corresponding structural properties of the nanorod arrays were investigated by scanning electron microscopy and X-ray diffraction.