Mechanical strength and electrical conductivity of Cu-Nb microcomposite wires with Cu core were investigated by tensile tests and resistivity measurements. Microstructure and fracture were examined by scanning electron microscopy. The results show that the fracture mode changes from normal fracture to shear fracture from the fracture core to the edge. The additional Cu core optimizes the conductivity and strength. The strength of deformed wires increases greatly with the decreasing of Nb filament spacing by a power law. Obvious size effect shows up when the spacing is below 100 nm. At the same time, the strength increases faster at 77 K than that at RT with decrease of spacing. When the spacing is above 300 nm, the size effect is slight for the strength.