Flash memory system and garbage collection method therof

A flash memory system and a garbage collection method thereof are provided to perform garbage collection in a high speed as uniformly distributing erase period over the whole region of a flash memory. According to a garbage collection method of a flash memory system, garbage collection cost is calculated by applying weight to each of more than two coefficients. A hash table is constituted with the calculated garbage collection cost. A block having the lowest garbage collection cost in the hash table is searched. Garbage collection is performed with the searched block.