A mask for thin film deposition used in forming an organic thin film or a conductive layer in an organic light emitting device is disclosed. In one embodiment, the mask includes i) a base member, ii) a plurality of slits configured to penetrate through the base member, wherein the plurality of slits have a predetermined length and extend in a first direction, wherein the plurality of slits comprise an outermost slit positioned in an outermost in a second direction having a predetermined angle with respect to the first direction, and wherein the outermost slit comprises two sub-slits separated from each other and iii) a rib supporting part formed between and contacting the two sub-slits, wherein the rib supporting part extends from a rib which is adjacent to the outermost slit.