The strength and equation of state of niobium (Nb) have be investigated using quasi-hydrostatic x-ray diffraction with helium as pressure medium up to 41 GPa, as well as radial x-ray diffraction up to 70 GPa in a 2-fold paranomic diamond anvil cell (DAC) at ambient temperature. The quasi-hydrostatic x-ray diffraction data (XRD) yields a bulk modulus and its pressure derivative as K0=166(2) GPa with K0'=3.2(2). The ratio of t/G is found to remain constant above ~6 GPa, indicating that the niobium started to experience yield with plastic deformation at this pressure. Combined with independent constraints on the high-pressure shear modulus, we found that niobium sample could support a differential stress of ~1.26 GPa when it started to yield with plastic deformation at ~6 GPa under uniaxial compression. The differential stress in niobium ranges from 0.03 GPa to 1.26 GPa with pressure increasing from 2 GPa to 6 GPa and can be described as t=-0.557(94)+0.306(21)P, where P is the pressure in GPa. The second increasing of t values occurs after ~30 GPa, suggesting that strengthen of Nb with a strength of ~1.67 GPa. A maximum differential stress, as high as ~3.96 GPa can be supported by niobium at the high pressure of ~70 GPa.
