A data writing method for writing page data into a rewritable non-volatile memory module is provided, the rewritable non-volatile memory module has a plurality of physical blocks, and each of the physical blocks has a plurality of physical pages. The data writing method includes grouping the physical pages into a plurality of physical page groups according to write speed of each physical page. The data writing method also includes compressing the page data to generate compressed data and calculating a data compression ratio corresponding to the compressed data. The data writing method further includes writing the compressed data into one of the physical pages in a corresponding physical page group among the physical page groups according to the data compression ratio. Accordingly, the data writing method can effectively ensure the accuracy of data stored in the rewritable non-volatile memory module.