A method of manufacturing a non-volatie memory device includes forming a tunnel insulating layer on a substrate, forming a conductive pattern on the tunnel insulating layer, forming a lower dielectric layer on the conductive pattern, performing a first heat treatment process to density the lower dielectric layer, and forming a middle dielectric layer having an energy band gap smaller than that of the lower dielectric layer on the first heat-treated lower dielectric layer. The method further includes forming an upper dielectric layer including a material substantially identical to that of the lower dielectric layer on the middle dielectric layer, performing a second heat treatment process to densify the middle dielectric layer and the upper dielectric layer and forming a conductive layer on the second heat-treated upper dielectric layer.