The nanocrystalline tungsten powder with the minimum grain size of around 5 nm was fabricated by mechanical alloying. The powder refinement process and the influence of ball to powder ratio (BPR) on the morphology and microstructure of tungsten powder were investigated. The morphology and microstructure were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). Results revealed that the high energy ball milling process has four stages. Compared with the samples with the BPR of 10:1 and 4:1 under the same ball milling conditions, the samples with the BPR of 15:1 experience the whole four stages after 60 h ball milling. One key factor in determining the final morphology and microstructure of tungsten powder by ball milling was revealed. The model to illustrate the fundamental mechanism for refining the particle size was discussed. The graphical abstract shows the powder refinement process and its schematic diagram. The changes of particle size with different BPR at different milling time was also shown. The nanocrystalline tungsten powder with grain size ranging from 5–15 nm was successfully fabricated by ball milling with the BPR of 15:1. Display omitted •The effect of ball to powder ratio on W powder refinement process was revealed.•The ball-milled W powder refinement process includes four stages.•Only the W powder with ball to powder ratio of 15:1 experiences the whole stage.•The final particle and grain size decrease with the increase of ball to powder ratio.•Nanocrystalline W powder with grain size of 5 nm–15 nm was fabricated.