A method of operating a memory device that performs a plurality of program loops for a plurality of memory cells includes applying a first program pulse and a first verify pulse of a first program loop from among the plurality of program loops, counting a first off cell count by using an output based on the first verify pulse, determining a first verify skip period using the first off cell count, applying an N-th program pulse and a plurality of verify pulses in response to an end of the first verify skip period, counting a second off cell count by using an output based on the plurality of verify pulses, and determining a second verify skip period using the second off cell count.