Operation methods for a memory device is provided. An operation method for the memory device comprises programming the memory device as described in follows. Data are provided. The data comprise a plurality of codes. Each number of the codes is counted. Then, a mapping rule is generated according to each number of the codes. In the mapping rule, each of the codes is mapped to one of a plurality of verifying voltage levels which are sequentially arranged from low to high. After that, the data are programmed into the memory device according to the mapping rule.