Arrays of single crystal germanium nanowires were fabricated in a self-assembled manner by a hydrothermal method without any metallic catalyst or templates. SEM, TEM, HRTEM, SAED, EDS, FTIR analyses show that the germanium nanowires are composed of a core of germanium and a very thin GeO₂ shell, and have a smooth surface and straightforward body, uniform diameters, a rectangular growth sectional view, and a knife edge growth tip. In the growth process, the H⁺ and OH⁻ play an important assistant role, a role of oxidation-reduction in the growth tip. PL shows that the germanium nanowires have intensive purple light emission at about 360 nm and 396 nm, which may be come from nano-size effect of GeNWs and the neutral oxygen vacancy, respectively.