A leakage current occurring on a boundary of a trench isolation region and an active region can be prevented in a Metal Oxide Semiconductor (MOS) Field Effect transistor, and a fabricating method thereof is provided. The transistor includes the trench isolation region disposed in a predetermined portion of a semiconductor substrate to define the active region. A source region and a drain region are spaced apart from each other within the active region with a channel region disposed between the source region and the drain region. A gate electrode crosses over the channel region between the source region and the drain region, and a gate insulating layer is disposed between the gate electrode and the channel region. An edge insulating layer thicker than the gate insulating layer is disposed on a lower surface of the gate electrode around the boundary of the trench isolation region and the active region.