Semiconductor memory device including dummy memory cells and method of operating the same

A method of operating a semiconductor memory device including a plurality of cell strings coupled to dummy word lines and normal word lines includes performing a first sub-program operation on selected normal memory cells by sequentially applying first program pulses to a selected normal word line and performing a second sub-program operation on the selected normal memory cells by sequentially applying second program pulses greater than the first program pulses to the selected normal word line, wherein at least one of the dummy word lines is biased in a same manner as the selected normal word line whenever each of the first program pulses is applied to the selected normal word line.