A flash memory device controls a common source line voltage and performs a program verify method. A plurality of memory cells is connected between a bit line and the common source line. A data input/output circuit is connected to the bit line and is configured to store data to be programmed in a selected memory cell of the plurality of memory cells. The data input/output circuit maintains data to be programmed within the data input/output circuit during a program verify operation, and decreases noise in the common source line by selectively precharging the bit line based on the data to be programmed.