Well-aligned ultralong ZnO nanorod arrays with a length of10 µm have been synthesized on glass substrates using a preheating hydrothermal method.The diameter of the nanorods is in the range from 50 to 80 nm, and the aspectratio and alignment can be simply controlled by varying the preheating time.Based on the evolution of aspect ratio with preheating time, a possible growthmechanism was proposed. X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) show that the nanostructures are well oriented with thec-axis perpendicular to the substrate. The photoluminescence (PL) spectrum of theas-grown ZnO nanostructure reveals a near-band-edge (NBE) emission peak and a yellowemission, and the origin of yellow emission was confirmed to be from the absorbedhydroxyl group. The ultralong nanorod arrays made in solution have a desirablediameter, length, density and orientation for ordered nanodevice applications.