Abstract: The present invention provides a MOS transistor and a method for manufacturing the same. The MOS transistor includes: a SOI substrate comprising a silicon substrate layer, an ultra-thin BOX layer, and an ultra-thin SOI layer; a metal gate layer formed on the SOI substrate; and a ground halo region formed in the silicon substrate layer and beneath the metal gate layer. The method for manufacturing a MOS transistor comprises: providing a SOI substrate, which comprises a silicon substrate layer, an ultra-thin BOX layer, and an ultra-thin SOI layer: forming a dummy gate conductive layer on the SOI substrate and a plurality of spacers surrounding the dummy gate conductive layer, removing the dummy gate conductive layer to form a opening; performing an ion-implantation process in the opening to form a ground halo region in the silicon substrate layer; and forming a metal gate layer in the opening.