ZnO nanostructures with different morphology were successfully prepared via a hexamethylendiamine-assisted hydrothermal synthesis route by only adjusting reaction temperature. The prepared ZnO samples were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Results indicated that uniform flake-like ZnO nanoparticles with a similar thickness of about 70 nm and flower-like ZnO nanostructures assembled by prism-like nanorods were prepared at 120 °C and 180 °C, respectively. Possible mechanisms for the formation of ZnO with different morphologies were discussed. The photocatalytic properties of the as-prepared flake-like ZnO nanoparticles and flower-like ZnO nanostructures were studied.