A thin film deposition apparatus used to produce large substrates on a mass scale and improve manufacturing yield. The thin film deposition apparatus includes a deposition source; a first nozzle disposed at a side of the deposition source and including a plurality of first slits arranged in a first direction; a second nozzle disposed opposite to the first nozzle and including a plurality of second slits arranged in the first direction; and a barrier wall assembly including a plurality of barrier walls arranged in the first direction so as to partition a space between the first nozzle and the second nozzle.