A method for forming a gate dielectric layer is described. A silicon oxide layer is formed on a semiconductor substrate. Then, a first and a second nitrogen doping processes are performed in sequence to the silicon oxide layer using plasma comprising inert gas and gaseous nitrogen to form a gate dielectric layer. The first nitrogen doping process is performed at a lower power, a lower pressure and a higher inert gas to nitrogen gas ratio than those at the second nitrogen doping process. The combination of the deeper nitrogen distribution of the first nitrogen doping process and the shallower nitrogen distribution of the second nitrogen doping process produces a flatter total nitrogen distribution profile so that leakage current from electron tunneling through the gate dielectric layer can be reduced.