A mask assembly for thin film vapor deposition of a flat panel display is disclosed. In the mask assembly, a cleansing solution remaining in the gap between a unit mask and a frame after cleansing the mask assembly may be minimized. The mask assembly includes a frame and a plurality of unit masks. The frame includes a pair of first supporting portions and a pair of second supporting portions that surround an opening. The plurality of unit masks have at least one set of pattern openings, and the unit masks are fixed on the first supporting portion while having a tensile force applied thereto. The first supporting portion includes a first surface and a second surface. The unit masks are fixed on the first surface, and the second surface has a height difference to the first surface along a thickness direction of the unit masks from the first surface.