In this work, we report a simple bottom-up approach to synthesize seamless tungsten disulfide (WS2)-tungsten (W) heterojunction electrode for efficient hydrogen evolution reaction (HER). Through one step sulfidation of WO3/W, ultra-thin vertically-arranged WS2 flakes with high surface areas and stepped-structure as well as abundant defects can grow on the W substrate. The synthesized WS2/W electrode exhibits outstanding HER activity with an onset overpotential of as small as approximately 30 mV, and an operating potential of only 108 mV for 10 mA/cm(2), which is close to the state-of-the-art transition metal dichalcogenides (TMDs) HER electrocatalyst reported in the literature so far. The abundant exposed edges and defects of WS2 along with excellent electrical contact between WS2 and W substrate contribute to the high HER activity of WS2/W electrode reported in this work.