A method for fabricating a transistor of semiconductor is disclosed. A disclosed method comprises: forming an STI structure and a well region in a silicon substrate; forming a first dummy gate electrode including spacers and a first gate oxide layer on the well region; forming source and drain regions including an LDD structure around the first dummy gate electrode by using the first dummy gate electrode and the spacers as a ion implantation mask, and performing a thermal treatment; removing the first dummy gate electrode and the first gate oxide layer under the first dummy gate electrode; forming a second dummy gate electrode and a second gate oxide layer; forming a thin nitride layer and a PMD on the silicon substrate including the second dummy gate electrode; performing a CMP process for the thin nitride layer and the PMD until the top of the spacers is exposed; removing the second dummy gate electrode and the second gate oxide layer; forming a third gate oxide layer and polysilicon for a gate electrode; performing another CMP process until the top of the spacers is exposed; and additionally etching the upper portion of the gate electrode.