Cu‐Nb wire composites with 0.105, 0.148, and 0.182 volume fraction of Nb filaments were producedinsituand their mechanical properties measured as a function of filament size and interfilament spacing. The yield stress and the ultimate tensile strength increased with both niobium volume fraction and overall composite reduction. At room temperature, the ultimate tensile strength of the Cu–18.2 vol% Nb composite reduced by 99.999% in cross‐sectional area (100–200 A˚ filament thickness) reached the value of 2230 MN/m2(323 ksi) and further increased to 2850 MN/m2(413 ksi) when measured at 77 °K. These values are higher by a factor of 4 than the values predicted by the rule of mixtures based on the highest reported strength of both niobium and copper. The composite strength is as high as that of the best copper whiskers and is shown to closely approach the theoretical strength of the material. The anomalous increase in strength despite the low volume fraction of reinforcing filaments suggests that the filaments act primarily as barriers to the motion of matrix dislocations and that the strength of the filamentary material is only of secondary importance. This hypothesis is supported by microstructural obsevations (transmission and scanning electron microscopy) which reveal the deformation modes during composite fabrication and mechanical testing. The excellent transport properties (in both the normal and superconducting state) make these composites attractive as conductors for high‐stress applications.