The Cu-Nb multicore composite wires reinforced by high-strength and high-conductivity Nb tube were prepared by a bundling and drawing process. The composite wires with different sizes (Φ3.0 mm, Φ2.7 mm, Φ2.5 mm) were selected, and the core filament morphology as well as the microstructure of Cu-Nb interfaces with different sizes was characterized by SEM; then the evolution and the characteristics of the core filament and the microstructure of Cu-Nb interfaces under multi-size conditions were discussed. Finally, based on σ-ε and R-T curve, the evolution mechanism of property caused by size change was analyzed. Results show that when the diameter of the sample is Φ2.5 mm, the strength of composite wire approaches 1.1 GPa and the conductivity reaches 74%IACS