To program in a nonvolatile memory device, a memory block is provided with a plurality of sub blocks disposed in a vertical direction where the memory block includes a plurality of cell strings each including a plurality of memory cells connected in series and disposed in the vertical direction. A plurality of intermediate switching transistors are disposed in a boundary portion between two adjacent sub blocks in the vertical direction. Each of the plurality of intermediate switching transistors is selectively activated based on a program address during a program operation. The selectively activating each of the plurality of intermediate switching transistors includes selectively turning on one or more intermediate switching transistors in a selected cell string based on the program address.