The present invention discloses a memory system having a hybrid density memory. The memory system includes a plurality of storage spaces whereby the storage spaces have respective levels of endurance and each storage space has a plurality of blocks and pre-determined weighting factors corresponding to the levels of endurance of the storage spaces. After executing a command of erasing a specific block, the system records the erase in accordance with the weighting factor of the storage space to which the specific block belongs. Whereby, the erase counts of all the blocks of different storage spaces are able to reach respective levels of endurance as simultaneously as possible.