A wear leveling method for a flash is provided, wherein the flash memory includes a plurality of physical blocks grouped into at least a data area and a spare area. The method includes setting a first predetermined threshold value as a wear-leveling start value and randomly generating a random number as a memory erased count, wherein the random number is smaller than the wear-leveling start value. The method also includes counting the memory erased count each time when the physical blocks are erased and determining whether the memory erased count is smaller than the wear-leveling start value, wherein a physical blocks switching is performed between the data area and the spare area when the memory erased count is not smaller then the wear-leveling start value. Accordingly, it is possible to uniformly use the physical blocks, so as to effectively prolong a lifetime of the store system.