Pure tungsten powder with coarse grain was ball milled to obtain nanocrystalline tungsten powder. The morphology and microstructure were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) for investigating the process of refinement. Results revealed that the powder grain refinement process in terms of the grain morphology and microstructure includes four stages, and three kinds of grains with different grain size and structure were observed through the deformation of tungsten powder and the interaction and accumulation of dislocations. The minimum grain size of 5 nm of tungsten powder was fabricated. The possible mechanism of nanocrystalline tungsten formation process through ball milling was discussed. (C) 2017 Elsevier B.V. All rights reserved.