The present invention discloses a tunnel insulating layer in a flash memory device and a method of forming the same, the method according to the present invention comprises the steps of forming a first oxide layer on a semiconductor substrate through a first oxidation process; forming a nitride layer on an interface between the semiconductor substrate and the first oxide layer through a first nitridation process; forming a second nitride layer on the first oxide layer through a second nitridation process; forming a second oxide layer on the second nitride layer through a second oxidation process; and forming a third nitride layer on the second oxide layer through a third nitridation process.