ZnO nanorod array with vertical orientation was fabricated by a simple aqueous solution way in the mild condition. ZnO granular film coated on glass substrate was used as a seeds film for the growth of ZnO ordered array. A metastable supersaturation area where there was no homogeneous nucleation of ZnO in the bulk solution but heterogeneous nucleation on the ZnO granular film was found. It was proved that high-quality and dense ZnO nanorod array could be obtained under this condition. The microstructures of ZnO nanorod array were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) equipped with energy dispersive spectrometer (EDS). The results indicated that ZnO nanorod array nucleated and grew vertically on the ZnO granular film along the [002] direction. The diameter and length of ZnO rod was greatly affected by growth solution concentration, reaction temperature, and metastable supersaturation status. The growth mechanism was discussed in detail.