Provided are a thin-film deposition mask assembly and a method of fabricating an organic light-emitting display panel using the same. A first mask includes a plurality of first openings disposed in odd columns and a plurality of second openings disposed in even columns. A single second opening disposed in an nth even column, among the plurality of second openings, is disposed between two first openings arranged in a first direction corresponding to row direction transverse to column direction and disposed in at least one column of (n−1)th and (n+1)th columns, among the plurality of first openings, where n is a natural number equal to or greater than 2. In some embodiments, the row direction and the column direction may be perpendicular to each other. The first mask is prevented from sagging.