The present invention provides a device isolation method of a semiconductor memory device and flash memory device fabricating method using the same, which can prevent a bridge occurrence between cells. The present invention includes forming a nitride layer pattern defining a trench forming area on a semiconductor substrate, forming a spacer on a sidewall of the nitride layer pattern, forming a trench in the semiconductor layer by removing a portion of the semiconductor layer using the nitride layer pattern and the spacer as an etch mask, forming a device isolation layer filling up the trench, removing the nitride layer pattern and the spacer to complete the device isolation layer, forming a conductor layer over the substrate including the device isolation layer, planarizing the conductor layer and the device isolation layer to lie in a same plane, and forming an insulating layer over the substrate.