The diameter of well-aligned ZnO nanorod arrays (ZNAs) grown on Si substrates has been well controlled from 150 to 40 nm by the two-step chemical bath deposition method (CBD), i.e. substrate pre-treatment with spin coating to form a ZnO nanoparticles seed layer and CBD growth. The effects of ZnO nanoparticles density and diameter on the size and alignment of ZNAs were investigated in detail by atomic force microscope (AFM), X-ray diffraction (XRD), scanning electronic microscope (SEM), transmission electron microscope (TEM) and photoluminescence (PL). The results indicate that both diameter and density of ZnO nanoparticles, which were pre-coated on the substrates, will influence the size and alignment of ZNAs, but density will play a key role in determining the diameter of ZNAs when the density is higher than the value of 2.3 x 10(8) cm(-2). Moreover, only a strong UV peak at 385 nm appears in room-temperature PL spectrum for these samples, which indicates that as-synthesized ZnO nanorods have a perfect crystallization and low density of deep-level defects. (C) 2008 Elsevier B.V. All rights reserved.