The effects of deformation and heat treatment on the electrical resistivity and the tensile strength of nanostructured Cu/Cu-18%Nb composite wires are studied. The correlated behavior of electrical resistivity and strength depending on the mode of thermo-mechanical processing is ascertained. The main mechanism of strengthening and increasing the electrical conductivity, associated with the reduction in the size of the structure, is considered. A good agreement is shown between the average distances between the Nb filament measured by SEM and extracted using the inversion of the resistivity size effect model. On the other hand, direct calculations of the strength value in accordance with the most common modern models are not consistent with experiments. In conclusion, the reasons for this discrepancy are discussed.