The described technology relates generally to an OLED display and manufacturing method thereof. The OLED display includes a substrate, a thin film transistor on the substrate and including a semiconductor layer, a gate electrode, a source electrode, and a drain electrode, and an organic light emitting element coupled to the thin film transistor and including a pixel electrode, an organic emission layer, and a common electrode, wherein the semiconductor layer is formed of a polycrystalline silicon layer, and remnants and contaminants at a surface of the polycrystalline silicon layer are reduced or eliminated through an atmospheric pressure plasma treatment. The semiconductor layer is formed of a polycrystalline silicon layer where remnants and contaminants at the surface thereof are reduced or eliminated through an atmospheric pressure plasma treatment.