Ultrafine-grained samples were consolidated from Ni nanopowders with the nominal particle size of 50 and 100 nm by Hot Isostatic Pressing (HIP) and Spark Plasma Sintering (SPS). The higher nickel-oxide content and the smaller grain size of SPS-processed samples result in a higher yield strength at room temperature compared with HIP-processed specimen. It is found that during compression the dislocation density increases while the twins decay in both samples, indicating that the deformation is mediated mainly by dislocations. As a consequence of the higher oxide content, the flow stress of the SPS-processed samples saturates at small strain values while the HIP-processed specimen shows strain hardening even at the strain value of 0.35. After saturation of the flow stress for SPS-processed samples the deformation is most probably mediated rather by grain rotation or grain boundary-related mechanisms such as sliding and/or decohesion instead of dislocation motion.