The invention provides a data writing method. In one embodiment, a data storage device comprises a flash memory. First, the flash memory is directed to read a plurality of programming voltage values for data programming. The programming voltage values are then adjusted to obtain a plurality of adjusted programming voltage values according to difference bits between a plurality of stored data patterns corresponding to the programming voltage values. The adjusted programming voltage values are then sent to the flash memory. The flash memory is then directed to perform data programming according to the adjusted programming voltage values, wherein the data programmed according to the adjusted programming voltage values has a lower error bit rate than that of the data programmed according to the programming voltage values.