The impacts of surface conditions on the growth of Ge nanowires on a Si ͑100͒ substrate are discussed in detail. On SiO 2 -terminated Si substrates, high-density Ge nanowires can be easily grown. However, on H-terminated Si substrates, growing Ge nanowires is more complex. The silicon migration and the formation of a native SiO 2 overlayer on a catalyst surface retard the growth of Ge nanowires. After removing this overlayer in the HF solution, high-density and well-ordered Ge nanowires are grown. Ge nanowires cross vertically and form two sets of parallel nanowires. It is found that nanowires grew along ͗110͘ directions. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2968201͔One-dimensional semiconductor nanostructures have attracted much attention because of their potential applications in the design of nanoelectronic, photonic, and sensing devices.1 Due to their high mobility of electrons and holes, Ge nanowires show their potential application in high-speed field-effect transistors.2-4 Moreover, Ge nanowires are potentially useful for high-speed quantum computing because of long decoherence time due to the predominance of spin-zero 5,6 nuclei and the advantage of a large excitonic Bohr radius in Ge ͑24.3 nm͒, allowing for quantum confinement to be observed in relatively large structures 7,8 and at high temperatures. In order to realize these applications, controllable and high-quality nanowire growth is important.