1. A method of performing a program operation in a resistance-variable memory device comprising a plurality of resistance-variable memory cells, the method comprising: supplying at least one pulse current to a selected resistance-variable memory cell; sensing data stored in the selected resistance-variable memory cell by detecting an amount of a verify current flowing through the selected resistance-variable memory cell; determining whether the sensed data is identical to data to be programmed in the selected resistance-variable memory cell; and if the sensed data is determined not to be identical to the data to be programmed, supplying a pulse current to the selected resistance-variable memory cell, a resistance value of the selected resistance-variable memory cell being changed by heat corresponding to the pulse current and the pulse current being increased when the supplying is repeated.