The hydrogen reduction of tungsten trioxide and ammonium para-tungstate was studied by means of a high pressure type thermobalance in the range of 1 to 20 atm and X-ray analysis. Owing to the formation of intermediate oxides, the feature of the reduction curve was complicated. Therefore, analysis of the reduction curve by using a topochemical reaction model was difficult. The initiation temperature for the reduction of WO3 was about 400°C. The effect of pressured hydrogen was observed mainly in the reduction from WO3 to WO2 at lower temperatures such as 500 and 600°C. At a high temperature, 700°C, the reduction rate of WO2 to W was also increased and the complete reduction time was shortened to a half of the time obtained in the atmospheric pressure.Para-tungstate began to decompose at about 50°C and changed to WO3 at around 400°C. Since then, the reduction from WO3 to W occurred. The reduction under high pressure has a merit to decrease the sublimation loss at about 800°C. Furthermore, the change of the particle shape in the reduction process was observed by scanning electron micrography, and the experimental results were discussed on the basis of an equilibrium diagram.