Three tungsten powder samples—one coarse grained (c-W; grain size: 1 μm–3 μm) and two nanocrystalline (n-W; average grain sizes: 10 nm and 50 nm)—are investigated under nonhydrostatic compression in a diamond anvil cell in separate experiments, and their in situ X-ray diffraction patterns are recorded. The maximum microscopic deviatoric stress in each tungsten sample, a measure of the yield strength, is determined by analyzing the diffraction line width. Over the entire pressure range, the strength of tungsten increases noticeably as the grain size is decreased from 1 μm–3 μm to 10 nm. The results show that the yield strength of tungsten with an average crystal size of 10 nm is around 3.5 times that of the sample with a grain size of 1 μm–3 μm.