A data writing method, a memory controller using the method, and a memory storage apparatus using the method are provided. The method includes selecting a physical block as a reserved physical block for a plurality of updated physical blocks. The method also includes, when a host system is about to write updated data into a logical page belonging to a logical block and a physical page, which corresponds to the logical page, of a substitute physical block, which corresponds to an updated physical block mapped to the logical block, has stored data, independently assigning the reserved physical block to the updated physical block mapped to the logical block and writing the updated data into the reserved physical block. Accordingly, the method can complete data writing without performing a data merge operation, thereby shortening the time for performing a write command.