Defects in a thin (500 Å) gate oxide, grown after the LOCOS process, are found to be distributed everywhere in the gate area as well as along the edge of the LOCOS pattern. The defect density is found to depend strongly on various process conditions of the LOCOS process, especially on the ambient gases in field oxidation. A new model for defect-inducing is proposed and examined by several experiments. The model is based in 1) diffusion of NH3 gas, produced at the top of the silicon nitride Si3N4 by the reaction of Si3N4 with H2O, to the SiO2–Si interface through Si3N4–SiO2; and 2) on the formation of nitride by the reaction of diffused NH3 with silicon substrates. Treatment by preliminary oxidation and etching-off of the oxide before gate oxidation is shown to be effective for removing the nitride induced by the LOCOS process at the SiO2–Si interface.