The present invention provides a memory size determining method which includes: writing a magic string into an initial location of the memory space of a memory; performing a first time reading with a first range on the memory, and if the magic string is not found in the first time reading, performing a second time reading with a second range on the memory, until the magic string is found; and if the magic string is found in the N-th time reading, determining the N-th range corresponding to the N-th time reading as the memory size, wherein N is an positive integer larger than or equal to 1.