A 3D memory device and a method of manufacturing the same, the device including a substrate including a cell and an extension region; a cell stack including insulation layers and word lines alternately stacked on the substrate; channel structures vertically passing through the cell stack; a word line separation layer vertically passing through the cell stack and extending lengthwise in a first direction; a contact plug vertically connected to the word lines on the extension region; and a bit line extending lengthwise in a second direction on the channel structures, wherein each of the word lines includes an inner pattern including polysilicon; and an outer pattern including metal, the outer pattern surrounds an outer surface of the inner pattern, the channel structures vertically pass through the inner pattern, and the contact plug is on the outer pattern.