A repair layer forming process includes the following steps. Firstly, a substrate is provided, and a gate structure is formed on the substrate, wherein the gate structure at least includes a gate dielectric layer and a gate conductor layer. Then, a nitridation process is performed to form a nitrogen-containing superficial layer on a sidewall of the gate structure. Then, a thermal oxidation process is performed to convert the nitrogen-containing superficial layer into a repair layer. Moreover, a metal-oxide-semiconductor transistor includes a substrate, a gate dielectric layer, a gate conductor layer and a repair layer. The gate dielectric layer is formed on the substrate. The gate conductor layer is formed on the gate dielectric layer. The repair layer is at least partially formed on a sidewall of the gate conductor layer.