We have investigated the structures of copper nanowires encapsulated in carbon nanotubes using a structural optimization process applied to a steepest descent method. Results show that the stable morphology of the cylindrical ultra-thin copper nanowires in carbon nanotubes is multi-shell packs consisted of coaxial cylindrical shells. As the diameters of copper nanotubes increases, the encapsulated copper nanowires have the face centered cubic structure as the bulk. The circular rolling of a triangular network can explain the structures of ultra-thin multi-shell copper nanowires encapsulated in carbon nanotubes.