A simple and efficient template-free electrochemical route has been utilized to prepare ultra-long Cu nanowires at a steady water-oleic acid interface. The as-prepared nanowires have uniform diameters and aspect ratios of more than 1000, and periodically twinned structure. The investigation of the growth process has indicated that these nanowires actually evolve from the branches of dendrites. The morphology of the products can be tuned by controlling experimental parameters, such as electrodeposition time, metallic ions concentration, current density, the geometry of electric field and the property and state of the interfaces. The experimental results show that the steady water-oleic acid interface and a parallel electric field are crucial for the formation of dendritic Cu nanowires. It is believed that the higher Cu(2+) concentration near the water-oleic acid interface is favorable to induce interfacial electrodeposition. The electrochemical route is also suitable for the synthesis of other metal nanowires, such as Co nanowires.