Flash memory device with a function for changing selectively size of memory cell block in erase operation and erase operation method of the same

A flash memory device and an erase operation method thereof are provided to change the size of a memory cell block during an erase operation, by changing the number of global word lines supplied with a voltage according to block size change signals without changing the physical structure. In a flash memory device, a plurality of memory cell blocks(MB1~MBK) includes a plurality of pages, respectively, and each of the pages includes a plurality of memory cells. An X-decoder(104) decodes block address signals, page address signals and block size change signals, in response to one of a program command, a read command and an erase command, and generates a plurality of block selection signals and word line bias voltages according to the decoding result, and outputs the word line bias voltages to a plurality of global word lines, respectively. A plurality of block selection parts(BS1~BSK) is arranged in the memory cell blocks, respectively, and selects each memory cell block, by connecting the global word lines, a global drain selection line and a global source selection line to the memory cell blocks, in response to the block selection signals. At least one block selection part selects at least one of the memory cell blocks, during the erase operation of the flash memory device. The X-decoder outputs the word line bias voltages to erase a part or all of the pages during the erase operation.