Well-aligned ZnO nanorod arrays had been successfully prepared by using hydrothermal methods. XRD and SEM analysis showed ZnO nanorod arrays had well orientation along the [0 0 1] direction of ZnO nanorods. The length to diameter is biggest among these samples and is about 25 when solution concentration is 0.8mol/l. The UV emission caused mainly by exciton combination. Furthermore the formation mechanism of ZnO nanorob arrays was studied in detail.