Methods of forming a semiconductor device having a metal gate electrode include sequentially forming a gate insulator, a gate polysilicon layer and a metal-gate layer on a semiconductor substrate. The metal-gate layer and the gate polysilicon layer are sequentially patterned to form a gate pattern comprising a stacked gate polysilicon pattern and a metal-gate pattern. An oxidation barrier layer is formed to cover at least a portion of a sidewall of the metal-gate pattern.