A method for fabricating metal gate transistor is disclosed. The method includes the steps of: providing a substrate, wherein the substrate comprises a transistor region defined thereon; forming a gate insulating layer on the substrate; forming a stacked film on the gate insulating layer, wherein the stacked film comprises at least one etching stop layer, a polysilicon layer, and a hard mask; patterning the gate insulating layer and the stacked film for forming a dummy gate on the substrate; forming a dielectric layer on the dummy gate; performing a planarizing process for partially removing the dielectric layer until reaching the top of the dummy gate; removing the polysilicon layer of the dummy gate; removing the etching stop layer of the dummy gate for forming an opening; and forming a conductive layer in the opening for forming a gate.