A mask assembly for thin film deposition, the mask assembly, including a mask frame having an opening; a mask coupled to the mask frame and having a first surface facing a substrate and a second surface opposite to the first surface, a plurality of deposition pattern portions, and a rib between the adjacent deposition pattern portions, the mask having a partially etched area, the plurality of deposition pattern portions being within the partially etched area; and a partial etching extension portion in an outer portion of an area corresponding to the plurality of deposition pattern portions.