A semiconductor memory device, an erasing method and a programming method are provided. The semiconductor memory device includes a memory array, which includes a plurality of NAND strings; a page buffer/sensing circuit, which is connected to the NAND strings of the memory array through bit lines and outputs whether the NAND strings include failures; and a detecting circuit, which is connected to the plurality of page buffer/sensing circuits and detects a number of the failures among the NAND strings of a selected block. The block is determined to be usable when the number of the failures among the NAND strings detected by the detecting circuit is less than or equal to a fixed number, and the block is determined to be unusable as a bad block when the number of the failures exceeds the fixed number.