Disclosed is a flash memory device and a program verification method thereof which can prevent a misjudgment as to whether flash memory cells are programmed or not. The flash memory device includes: a program verification voltage generator for variably generating program verification voltages used to verify whether the flash memory cells are programmed or not and a word line level selector for transferring the program verification voltages to word lines connected to control gates of the flash memory cells. The flash memory cells that are verified as uncertain as to whether the flash memory cells are programmed or not can be completely programmed since the program verification operation is carried out with program verification voltage levels that are changed according to the selective activations of the program verification control signals.