The present invention relates to a method of manufacturing a flash memory device. In the method, a low-voltage transistor is formed to have a DDD structure same to a high-voltage transistor when a peripheral region is formed in the manufacture process of the flash memory device. As the process for forming the LDD structure for the low voltage is omitted, the cost is reduced in the entire process of manufacturing the flash memory device. Also, as the junction breakdown voltage of the low-voltage transistor is increased and current is increased, the device characteristics is improved