In this study, well-aligned single-crystalline zinc oxide (ZnO) nanorods arrays were synthesized on Si substrate by simple hydrothermal process in dilute aqueous solution which consisting of zinc nitrate tetrahydrate (Zn(NO3)2•4H2O) and hexamethyltetramine (C6H12N4, HMT) at 90 °C, in the autoclave. ZnO thin film was pre-deposited on Si substrate by the sputter deposition to serve as the seed layer. Aluminum nitrite (Al(NO3)3) also has been mixed into reaction solution. Al doped ZnO nanorod arrays were fabricated successfully. ZnO nanorods were characterized by high resolution X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, and photoluminescence measurement.