A semiconductor device and a method of forming the same are provided. The semiconductor device includes a gate trench crossing an active region, and a gate structure in the gate trench. The gate structure includes a gate dielectric layer disposed on an inner wall of the gate trench, a gate electrode disposed on the gate electric layer and partially filling the gate trench, a gate capping insulating layer disposed on the gate electrode, and a gap-fill insulating layer disposed in the gate trench and disposed on the gate capping insulating layer. The gate capping insulating layer includes a material formed by oxidizing a portion of the gate electrode, nitriding the portion of the gate electrode, or oxidizing and nitriding the portion of the gate electrode.