A storage device for performing a garbage collection operation using a partial block erase operation includes: a memory device including a plurality of main blocks each including a plurality of sub-blocks; and a memory controller configured to perform a garbage collection operation for securing free blocks in which no data is stored, among the main blocks, wherein the memory controller includes a write handler configured to erase at least a portion of a target block, among the main blocks, according to whether an amount of valid data in at least one victim block exceeds a storage capacity of one main block.