Effect of oxygen content in the ultrafine tungsten powder fabricated by electrical explosion of wire method on the behvior of spark plasma sintering was investigated. The initial oxygen content of 6.5 wt% of as-fabricated tungsten powder was reduced to 2.3 and 0.7 wt% for the powders which were reduction-treated at $400^{circ}C$ for 2 hour and at $500^{circ}C$ for 1h in hydrogen atmosphere, respectively. The reduction-treated tungsten powders were spark-plasma sintered at $1200-1600^{circ}C$ for 100-3600 sec. with applied pressure of 50 MPa under vacuum of 0.133 Pa. Maximun sindered density of 97% relative density was obtained under the condition of $1600^{circ}C$ for 1h from the tungsten powder with 0.7 wt% oxygen. Sintering activation energy of $95.85kJ/mol^{-1}$ was obtained, which is remarkably smaller than the reported ones of $380{sim}460kJ/mol^{-1}$ for pressureless sintering of micron-scale tungsten powders.