A method for fabricating a MOS device is described. A first hard mask layer is formed over a substrate. The first hard mask layer is patterned and a portion of the substrate removed to form a first patterned hard mask, and a fin structure surrounded by a trench and extending in a first direction. An insulating layer is formed at the trench bottom. A gate conductive layer is formed on the insulating layer, extending in a second direction. A first implant process is performed using the first patterned hard mask as a mask to form first S/D extension regions in the sidewalls of the fin structure. The first patterned hard mask is removed to expose the top of the fin structure, and then a second implant process is performed to form second S/D extension region therein.