Abstract: By using a low temperature hydrothermal approach, well-aligned ZnO nanorod arrays were pre-pared on substrates, which were pre-treated with colloid pre-treating method in different growth periods of time. Scanning electron microscopy and X-ray diffraction spectroscopy were employed to study the morphology of ZnO nanorod arrays. Kinetic studies show that the first 8 h is the most important growth period, beyond which the nanorods nearly stop growing. During the first 8 h, the growth of width of ZnO nanorods contains two distinct steps:a fast step within the first 1.5 h, in which the nanorods tend to be short and wide, followed by a slow step, in which long rods with high aspect ratio are obtained. The length of ZnO nanorod arrays, which essentially represents the thickness of a homogeneous monolayer of the thin film, may be experimentally tailored to any required dimension of up to 2.4 μm at a growth rate of approximately 5.5 nm·min^-1.