A flash memory device includes at least two mats and a row decoder shared by the mats. Each mat includes multiple word lines, bit lines, and blocks that share the bit lines. The row decoder includes a block decoder that generates a block selection signal for selecting a block, a block word line boosting circuit that generates a high voltage block word line signal in response to the block selection signal, a word line driver that drives word line drive signals driving the word lines of the selected block using drive voltages according to an operation mode and the word lines of an unselected block using a first bias voltage, and a string selection line driver that drives a string selection signal of the selected block using a drive voltage according to the operation mode and the string selection signal of the unselected block using a second bias voltage.