The invention provides a method for improving the reliability of a gate oxide layer of a peripheral circuit area of a flash memory. The method includes the following steps that: oxidation treatment is performed on a nitrogen containing region of the top of a semiconductor substrate at the peripheral circuit area; and the nitrogen containing region is converted into an oxide layer, and the oxide layer is removed, and therefore, a nitrogen-free semiconductor substrate surface of the peripheral circuit area is exposed; and a gate oxide layer is grown on the nitrogen-free semiconductor substrate surface. With the method adopted, when an MOS device is manufactured in the peripheral circuit area in a subsequent process, the nitrogen containing region of the top of the semiconductor substrate is removed, and therefore, the growth difficulty of the gate oxide layer on the surface of the semiconductor substrate can be lowered, and especially, the growth capacity of the gate oxide layer on the semiconductor substrate at corners of an isolation structure can be improved, and the integrity and uniformity of gate oxide layer growth can be increased, and the reliability of the gate oxide layer of the MOS device can be improved; and the oxidation treatment mentioned in the invention is performed at room temperature or low temperature, and therefore, thermal budget in the prior art can be decreased, and device doping profile shift can be avoided. The method is advantageous in convenient implementation and simple operation.