A method for fabricating a metal gate transistor is disclosed. The method includes the steps of: providing a substrate having a first transistor region and a second transistor region; forming a first metal-oxide semiconductor (MOS) transistor on the first transistor region and a second MOS transistor on the second transistor region, in which the first MOS transistor includes a first dummy gate and the second MOS transistor comprises a second dummy gate; forming a patterned hard mask on the second MOS transistor, in which the hard mask includes at least one metal atom; and using the patterned hard mask to remove the first dummy gate of the first MOS transistor.