Superhard materials such as diamond and cubic boron nitride (cBN) are becoming ever more scientifically and technologically important, and critical and fundamental knowledge about their constitutive properties and deformational mechanisms is in increasingly high demand. Although it has long been suggested by theoretical modeling that deformation of face-centered cubic superhard materials is dominated by Shockley partial dislocations and screw dislocations, there has been a glaring lack of experimental evidence. Here, we report in situ deformation experiments of nanocrystalline cBN (nc-cBN) samples at high pressures and temperatures using a deformation-DIA (D-DIA) apparatus coupled with synchrotron X-ray diffraction techniques. Intrinsic stress-strain relations have been obtained for nc-cBN for the first time, and only elastic deformation occurred up to a strain of at least 14% at room temperature (RT), demonstrating its remarkable strength, which was undoubtedly enhanced by observed microscopic features such as the Lomer-Cottrell (L-C) locks and high-angle GBs. While deformation at RT is dominated by brittle fractures and mechanic crushing induced by grain boundary twisting mediated by full dislocations, plasticity of nc-CBN at higher temperatures is controlled by grain rotation and twinning mediated by Shockley partial dislocations. At 4.0 GPa and 1200 °C, accumulated shear strain resulted in the conversion of cBN to hBN at or near twisting GBs, releasing stress and mediating deformation in the process. We demonstrate the apparent agreement between the differential micro-stress derived from peak broadening analysis and differential macro-stress deduced from lattice strain analysis.