A mask assembly configured to prevent the mask from being thermally transformed and to maintain gaps between a plurality of divided pattern masks is disclosed. According to one embodiment, the mask assembly comprises: an open mask that has a plurality of first openings arranged in rows and columns; and a pattern mask that comprises a plurality of unit pattern masks. In some embodiments, the ends of the unit pattern masks may be fixed with respect to the open mask, and the unit pattern mask includes a plurality of mask pattern units configured to align with the plurality of first openings of the open mask. In some embodiments, a tensile force is applied to the unit pattern mask(s).