Scheelite ores endolithic arsenic (As) is commonly removed as a detrimental impurity element in the process of tungsten metallurgy. However, arsenic can refine tungsten grains significantly which was found by our team. In this study, refining mechanisms of arsenic in the hydrogen reduction process of tungsten oxide were discussed in detail by X-ray diffraction, scanning electron microscope and transmission electron microscope. The results showed that the WAs2 intermediate phases played an important role as heterogeneous nucleation core for tungsten during the nucleation process. Moreover, the process of chemical vapor transport was hampered by volatilization of arsenic, which inhibited the growth of tungsten particles. The above two effects resulted in the W-1% As composite powder with the average size of about 80 nm in diameter. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.