Disclosed is a thin film transistor including a P-type semiconductor layer, and an organic light-emitting display device having the thin film transistor. The present invention provides a thin film transistor including a substrate, a semiconductor layer, and a gate electrode and a source/drain electrode formed on the substrate, wherein the semiconductor layer is composed of P-type ZnO:N layers through a reaction of a mono-nitrogen gas with a zinc precursor, and the ZnO:N layer includes an un-reacted impurity element at a content of 3 at % or less.