A paper cassette of an image forming apparatus including: a cassette body containing sheets of paper, and a paper alignment apparatus mounted in the cassette body. The paper alignment apparatus including: an alignment apparatus body having a paper alignment surface contactable with corners of one side of sheets of paper and slidable in a horizontal direction, and a locking unit mounted in the alignment apparatus body and locked into the cassette body. The locking unit is downwardly pressed when being locked and unlocked.