The process of high-pressure torsion (HPT) was applied to control the size and distribution of ferromagnetic Co particles in a Cu-Co alloy. Electron probe microanalysis, X-ray diffraction analysis, and transmission electron microscopy confirmed that the Co particles were significantly refined through fragmentation and dissolved with intense straining by HPT. Magnetoresistance appeared by similar to 2.5% at 77 K with an isotropic feature corresponding to giant magnetoresistance (GMR). It is demonstrated that HPT is a potential process for creating GMR in the Cu-Co alloy prepared by conventional ingot metallurgy.