A comprehensive mechanistic study conducted on the formation mechanism of five-fold twinned copper nanowires by heating copper(I) chloride with oleylamine at 170 °C is presented. Electron microscopy and UV-visible absorption spectra are used to analyze the growth mechanism of copper nanowires. High-resolution transmission electron microscopy and selected-area electron diffraction are used to investigate the detailed structure of copper nanowires and nanoparticles, and a five-twinned structure is shown to exist in the copper nanowires and nanoparticles. Additionally, experiments have been performed to indirectly confirm that oleylamine preferentially adsorbs on the {100} facets of growing crystals. On the basis of the above results, the self-seeded growth of copper nanowires is confirmed. In the initial stage of reactions, copper nanoparticles with two distinctive sizes are formed. As the reaction proceeds, larger five-twinned copper nanoparticles serve as seeds for anisotropic crystal growth. Further, copper atoms generated from an Ostwald ripening process or reduction reactions of a copper(I) chloride-oleylamine complex continue to deposit and crystallize on the twin boundaries. Once the {110} planes are generated, oleylamine preferentially adsorbs on the newly formed {100} facets and then guides the formation of nanowires. The electrical resistivity of a single copper nanowire is measured to be 41.25 nΩ-m, which is of the same order of magnitude as the value of bulk copper (16.78 nΩ-m). Finally, an effective surface-enhanced Raman spectroscopy active substrate made of copper nanowire is used to detect the 4-mercaptobenzoic acid molecules.