A MOS transistor suppressing a short channel effect includes a substrate, a first diffusion region and a second diffusion region separated from each other by a channel region in an upper portion of the substrate, a gate insulating layer including a first gate insulating layer disposed on a surface of the substrate in the channel region and a second gate insulating layer having a specified depth from the surface of the substrate to be disposed between the first diffusion region and the channel region, and a gate electrode disposed on the first gate insulating layer.