Amorphous silver nanowires (a-Ag NWs) are fabricated from crystalline Ag NWs by using 5 MeV helium (He+) ion beam irradiation. At low dose (5×1015 ion/cm2), few defects are created in Ag NWs. As dose increases, more damage to the crystalline structure of Ag NWs is observed. Finally at high dose (8×1016 ion/cm2), the face-centered cubic structure of Ag NWs is transformed into the amorphous structure with similar morphology as Ag NWs. Phase transformation of crystalline Ag NWs upon irradiation with 5 MeV He+ ions is observed through high resolution transmission electron microscopy. Synthesis of large scale amorphous metal nanowires and metal nanowire alloy systems are discussed.