In this study, the effects of growth temperature on structural and optical properties of hydrothermally grown ZnO nanorod arrays have been investigated. Zinc nitrate ($Zn(NO_3)_2$) and hexamethylenetetramine were used as precursors. The ZnO buffered Si(100) with a thickness of 40 nm was used as the substrates. The ZnO nanorods were grown on these substrates with the temperature ranging from 55 to $115^{circ}C$. The results were characterized by scanning electron microscope, X-ray diffraction and room temperature photoluminescence measurements. Well-aligned ZnO nanorods arrays were obtained from all samples. The tips of nanorods were flat when the temperature was less than $95^{circ}C$, and the sharp-tip nanoneedle-like morphologies were obtained with the temperature of $115^{circ}C$. In addition, some bundles were on the nanorods arrays with $115^{circ}C$ due to the non-equilibrium growth. The growth temperature could affect the crystal and optical properties of ZnO. For the effects on crystal properties, the intensity of (002) peak was increased as the temperature was increased to $75^{circ}C$, then decreased as the temperature was further increased to $115^{circ}C$. As for the effects on optical properties, the intensity ratio of UV peak to visible peak is increased with the temperature increasing and the strongest UV peak intensity was obtained with the growth temperature of $95^{circ}C$.