A method of manufacturing a flash memory device may include forming a trench, defining at least a common source region, on a semiconductor substrate, forming a gate poly over the semiconductor substrate, performing an ion implantation process employing a first photoresist pattern and the gate poly as a mask, wherein the ion implantation process forms a source/drain junction on the semiconductor substrate, forming a recess common source region in the trench by using a second photoresist pattern, and performing an ion implantation process on the recess common source region.