We report a simple method, oblique angle deposition, to directly synthesize aligned Ge nanowire arrays on a Si substrate. This process is accomplished by tilting the Si substrate and adjusting the incident angle of the evaporated Ge vapor flux with respect to the substrate normal to 87°. The resultant crystallinity of the Ge nanostructures can be tuned to either amorphous or poly- and single-crystalline, depending on the substrate temperature and evaporation rate. The effects of thermal treatment on the morphology and structure of the Ge nanowires are discussed in detail. The field-emission measurements show that increasing the annealing temperatures to about 550 °C results in a gradual increase in the maximum current density and a decrease in the turn-on voltage, because of the decreased wire density originating from melting of the Ge nanowires. The field-enhancement factor analysis shows there is an optimum range for Ge wire density and aspect ratio to obtain good emission performance. Ge nanowire arrays might find potential application in the field emitters of the future.