A nonvolatile memory device includes at least one memory block and a control circuit. The at least one memory block includes a plurality of cell strings that are divided into a plurality of stacks disposed in the vertical direction, and each of the plurality of stacks includes at least one dummy word-line. The control circuit controls a program operation by applying a program voltage to a selected word-line of the plurality of cell strings during a program execution period and by reducing a voltage level of a dummy voltage applied to the at least one dummy word-line of at least one upper stack from among the plurality of stacks during the program execution period. The at least one upper stack is disposed at a higher position than a selected stack in the vertical direction and the selected stack from among the plurality of stacks includes the selected word-line.