Magnetoresistance, antiferromagnetic coupling, and crystallographic orientation of Co/Cu superlattices with intentionally mixed interfaces have been studied as a function of the thickness of the mixed region. The antiferromagnetic coupling is weakened, and spin‐independent scattering of free electrons is enhanced with increasing thickness of the mixed region, although the morphology and the superlattice period remain unchanged. Saturation magnetoresistance is reduced from 27% to 4% as the result of the formation of a 0.15 nm mixed region at the interfaces. Moreover, the crystallographic orientation of Co/Cu superlattices is also found to be varied by formation of the mixed region. Giant magnetoresistance, antiferromagnetic coupling, and the crystallinity of Co/Cu superlattices are governed by the events in the thin region at the interfaces less than 1 monolayer.