A nonvolatile memory device has improved reliability by compensating a threshold voltage of a flash memory cell. A nonvolatile memory device includes a memory cell array and a voltage generator for supplying a select read voltage to a select word line and an unselect read voltage to unselected word lines when a read operation is performed, and supplying a verify voltage to a select word line and the unselect read voltage to unselected word lines when a program operation is performed. The voltage generator supplies a first unselect read voltage to at least one between an upper word line and a lower word line adjacent to the select word line when the program operation is performed, and supplies a second unselected read voltage to at least one between the upper word line and the lower word line adjacent to the select word line when the read operation is performed.