ZnO nanorods (NRs) were hydrothermally synthesized by using equimolar zinc nitrate hydrate (Zn(NO3)26H2O) and hexamethylenetetramine (C6H12N4) solutions. The shape of the nanostructures, obtained by aqueous method, was greatly influenced by the growth temperature and the molar concentrations. NRs grown at higher temperature (90C) have rounded tips, whereas nanostructures of hexagonal flat-end shape were obtained at 75C. Hardly any nanostructures were observed by further reducing the temperature to 60C. In addition, solutions with higher molarity favored the appearance of nanoflowers. Scattered ZnO NRs were observed on silicon substrate, whereas aligned ZnO nanowires (NWs) 50-70nm in diameter were obtained at 75C by introducing sputtered ZnO film as a seed layer. High-resolution transmission electron microscopy (HRTEM) confirmed the growth of ZnO nanowires along [001] direction. A band-edge luminescence along with a broad visible spectrum was observed for the ZnO nanowires.