A program method of a memory device include determining whether valid data is stored in memory cells of a word line adjacent to a selection word line upon which a program operation is to be performed; when the valid data is not stored in the memory cells of the word line adjacent to the selection word line, performing, based on data to be written to the selection word line, a pre-program operation on the word line adjacent to the selection word line; and after the performing of the pre-program operation, performing, based on a program command, the program operation on the selection word line.