A nonvolatile semiconductor device according to example embodiments may include a plurality of memory cells on a semiconductor substrate and at least one selection transistor on the semiconductor substrate, wherein the at least one selection transistor may be disposed at a different level from the plurality of memory cells. The at least one selection transistor may be connected to a data line and/or a power source line via a first contact and/or a third contact, respectively. The at least one selection transistor may be connected to the plurality of memory cells via a second contact and/or a fourth contact. The active layer of the at least one selection transistor may contain an oxide. Accordingly, the nonvolatile semiconductor device according to example embodiments may include a selection transistor having a reduced size.