A method for forming metal lines of a semiconductor device is disclosed. The metal line forming method includes forming plugs by perforating via-holes in an interlayer dielectric layer formed on a semiconductor substrate and burying a conductive material in the via-holes, sequentially forming at least two metal layers on the interlayer dielectric layer formed with the plugs, the metal layers having a difference in the size of metal grains of each metal layer, etching an uppermost first metal layer of the at least two metal layers using a photoresist pattern formed on the first metal layer as an etching mask using a first etching gas, and etching the partially etched first metal layer using a second etching gas.