A highly sensitive hydrogen sensor based on suspended and functionalized single tungsten nanowire is presented. The nanowire sensing structure is formed by Focused Ion Beam-Chemical Vapor Deposition on the batch-fabricated microelectrodes. The tungsten nanowire measures 10μm in length and 150nm in diameter, and is suspended 50μm above the wafer substrate connecting two microelectrodes. The nanowire is then functionalized by Pd-Pt metallization to be sensitive to hydrogen gas. The experimentally characterized hydrogen sensor shows 11.52% of resistance increase to the exposure to 10000ppm (1%) hydrogen gas and 99% nitrogen gas mixture at room temperature and atmospheric pressure. The lowest hydrogen concentration we measured is 10ppm with 0.58% corresponding resistance increase.