A method of programming a memory system includes repetitively performing N program loops for a selected memory cell (where N is a natural number equal to or greater than two). Each of the N program loops includes a program operation and a program verify operation. At least one of the N program loops includes performing the program operation on the selected memory cell and on at least one additionally selected memory cell by applying a program voltage to at least one word line to which the selected memory cell and at least one additionally selected memory cell are connected, and performing the program verify operation on the selected memory cell by applying a program verify voltage to a selected word line to which the selected memory cell is connected.