A mask stack and a method of controlling the mask stack are disclosed. In one aspect, the method includes installing, in the mask stack, a first cassette comprising masks that have not been used in a deposition process and a second cassette comprising an accommodation space which is empty. The method also includes using, in the deposition process, at least one mask from the masks included in the first cassette. The method further includes inserting, into the accommodation space of the second cassette, the at least one mask used in the deposition process.