A program method of a nonvolatile memory device includes programming data of a first bit into a target page of a plurality of pages in a memory cell array, sensing the programmed data and storing the sensed data in a page buffer coupled to the memory cell array, erasing data of the target page, inputting data of a second bit to the page buffer and generating program data by combining the data of the second bit and the data of the first bit stored in the page buffer, and programming the program data into the target page.