Highly oriented and uniform-sized Cu2S nanowire arrays grown on copper substrates have been studied by high-resolution transmission electron microscopy. Cu2S nanowires consist of single crystalline cores of Cu2S and polycrystalline Cu2O shells. Most Cu2S nanowires have a similar crystalline orientation and their growth directions were [001](Cu2S) as determined by electron diffraction and HRTEM. The orientation relationship between Cu2S crystalline core and Cu2S crystals in the shells was determined to be (100)(Cu2S)//(110)(Cu2O) and [010](Cu2S)//< 111 > (Cu2O.) The nucleation and growth process of Cu2S nanowires were interpreted by oxide-assisted growth mechanism.