A semiconductor structure is disclosed. The semiconductor structure includes a substrate, and an interlayer dielectric disposed on the substrate which has agate structure therein. The gate structure further includes a gate electrode with a protruding portion, and a gate dielectric layer disposed between the gate electrode and the substrate. A spacer is disposed between the interlayer dielectric and the gate electrode. An insulating cap layer is disposed atop the gate electrode and encompasses the top and the sidewall of the protruding portion.