Embodiments relate to a method for forming a gate insulating layer, which may include forming a device isolation layer being divided into a device active region and a device isolation region, growing a first oxide layer at an entire surface of the semiconductor substrate as a gate insulating layer, performing a first annealing process to form a diffusion barrier layer an interface between the first oxide layer and the device active region, etching and removing a first oxide layer and a diffusion barrier layer of the core power source wiring region by masking the input/output power source wiring region, growing a second oxide layer on the core power source wiring region, and performing a second annealing process to form an NO gate oxide layer on which an N-rich oxide layer at an interface of the core power source wiring region.