Method of manufacturing flash memory device

A method of manufacturing a flash memory device according to an embodiment includes forming a second oxide layer pattern having a mask pattern buried therein on a first nitride layer pattern and a first oxide layer stack on a semiconductor substrate; forming first polysilicon patterns at sidewalls of the buried mask pattern; removing portions of the first oxide layer, the first nitride layer pattern, and the second oxide layer pattern to form a third oxide layer pattern, a second nitride layer pattern, and a fourth oxide layer pattern at lower portions of the first polysilicon patterns and the mask pattern; forming a fifth oxide layer pattern surrounding each of the first polysilicon patterns; forming second polysilicon patterns on sidewalls of the fifth oxide layer pattern; and removing the mask pattern and parts of the third oxide layer pattern and the second nitride layer pattern between the first polysilicon patterns.