1. A method of performing a program operation in a phase change memory device comprising a plurality of phase change memory cells, the method comprising: (a) receiving program data to be programmed in selected memory cells among the plurality of phase change memory cells; (b) sensing read data stored in the selected memory cells by detecting respective magnitudes of verify currents flowing through the selected memory cells when a verify read voltage is applied to the selected memory cells; (c) determining whether the read data is identical to the program data; and (d) upon determining that the program data for one or more of the selected memory cells is not identical to the corresponding read data, programming the one or more selected memory cells with the program data.