A method for fabricating a gate dielectric layer comprises the steps of: forming a dielectric layer on a semiconductor substrate; performing a nitrogen treating process to form a nitride layer on the dielectric layer; and performing a thermal treating process at 1150-1400° C. for a period of 400-800 milliseconds, to form a gate dielectric layer. A step of forming a gate layer on the gate dielectric layer may be performed to form a gate structure.