Provided is a programming method of a nonvolatile memory device, the method comprising the steps of a first programming loop including applying a first verifying voltage to word lines of a plurality of first memory cells for being programmed in a first programming state of a first target threshold voltage and detecting, from among the plurality of first memory cells, a first slow memory cell whose threshold voltage is less than the first verifying voltage, a second programming loop including applying a first program pulse to the first memory cells and applying a second program pulse to the first slow memory cell, a voltage level of the second program pulse of the second program loop being greater than a voltage level of the first program pulse of the second program loop, and a third programming loop.