Large-yield and crystalline GaN nanowires have been synthesized on a Si substrate via a simple thermal evaporation process. The majority of the GaN nanowires has bicrystalline structures with a needlelike shape, a triangular prism morphology, and a uniform diameter of similar to 100 nm. Field-emission measurements show that the bicrystalline GaN nanowires with sharp tips have a lower turn-on field of similar to 7.5 V/mu m and are good candidates for low-cost and large-area electron emitters. It is believed that the excellent filed emission property is attributed to the bicrystalline structure defects and sharp tips.