Through varying the reducing agents, sphere-shaped UO₂ nanoparticles (average diameter 100 nm) consisting of 15 nm nanocrystal subunits and single crystalline U₃O₈ nanorods (diameter ranging from 80 to 100 nm) have been synthesized selectively under hydrothermal conditions. The nanorods, which grew along the crystallographic [001] direction, were found to be 500–1500 nm in length. By adjusting the reaction conditions such as the volume ratio of the solvent and the reducing agent, the size and morphology of the uranium dioxide nanostructures varied to a certain degree. The sphere-shaped UO₂ particles were converted to porous U₃O₈ aggregates through calcination at 800 °C for 2 h in air. The structure and morphology of the obtained products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Furthermore, a catalytic test on the porous U₃O₈ aggregates and the U₃O₈ nanorods indicates that the catalytic performance of the uranium oxide materials for the oxidation of benzyl alcohol is morphology-dependent.