The Ag-catalyzed growth of straight Ge nanowires (GeNWs), serrated Ge nanobelts (GeNBs), and hexagonal Ge nanotowers (GeNTs) by thermal evaporation of Ge powder at 950 degrees C in Ar was studied. The growth of GeNWs and GeNBs at 550-600 degrees C followed the top-growth mode via the vapor-solid-solid process, while that of GeNTs at 700-750 degrees C followed the bottom-growth mode via the vapor-liquid-solid process. This result shows that the growth mode of Ge nanostructures catalyzed by Ag nanoparticles is temperature-dependent. The larger size of AgGe droplets assembled at high temperatures is beneficial to the growth of GeNTs with the bottom-growth mode. In addition, the growth mechanisms of Ge nanostructures are discussed.