A semiconductor memory device has a memory cell region and a peripheral region. The device includes low voltage transistors at the peripheral region having gate insulation films with different thicknesses. For example, a gate insulation film of a low voltage transistor used in an input/output circuit of the memory device may be thinner than the gate insulation film of a low voltage transistor used in a core circuit for the memory device. Since low voltage transistors used at an input/output circuit are formed to be different from low voltage transistors used at a core circuit or a high voltage pump circuit, high speed operation and low power consumption characteristics of a non-volatile memory device may be.