A semiconductor memory device includes a substrate having a first region and a second region. A first gate electrode layer is on the first region and includes a first conductive layer including a first plurality of layers, and includes a first upper conductive layer on the first conductive layer. A second gate electrode layer is on the second region and includes a second conductive layer including a second plurality of layers, and includes a second upper conductive layer on the second conductive layer. At least one of the first plurality of layers includes titanium oxynitride (TiON). A first transistor including the first gate electrode layer and a second transistor including the second gate electrode layer are metal oxide semiconductor field effect transistors (MOSFETs) having the same channel conductivity type, and a threshold voltage of the first transistor is smaller than a threshold voltage of the second transistor.