Ultra-fine grained materials exhibit outstanding properties and are therefore favorable for prospective applications. One of these promising systems is the composite assembled by the body centered cubic niobium and the face centered cubic copper. Cu-Nb composites show a high hardness and good thermal stability, as well as a high radiation damage tolerance. These properties make the material interesting for use in nuclear reactors. The aim of this work was to create a polycrystalline ultra-fine grained composite for high temperature applications. The samples were manufactured via a powder metallurgical route using high pressure torsion, exhibiting a randomly distributed oriented grain size between 100 and 200. nm. The mechanical properties and the governing plastic deformation behavior as a function of temperature were determined by high temperature nanoindentation up to 500. °C. It was found that in the lower temperature regions up to 300. °C the plastic deformation is mainly governed by dislocation interactions, such as dislocation glide and the nucleation of kink pairs. For higher temperatures, thermally activated processes at grain boundaries are proposed to be the main mechanism governing plastic deformation. This mechanistic view is supported by temperature dependent changes in hardness, strain rate sensitivity, activation volume, and activation energy.