A mask assembly includes a mask and a blocking stick. The mask includes a pattern region formed therein with openings and extends in a first direction. The blocking stick is disposed under the mask and overlaps a first side portion of the mask. The mask further includes dummy opening regions and opening regions. The dummy opening regions are arranged in the first direction in the first side portion of the mask, and are formed therein with openings. The opening regions are arranged in the first direction in a second side portion which is opposite to the first side portion of the mask and correspond to the dummy opening regions, respectively. The number of the openings per a unit area in each of the dummy opening region and the opening region is smaller than the number of the openings per a unit area in the pattern region.