An organic light emitting display device includes a substrate, an active layer, a gate electrode, a first high dielectric constant (hereinafter “high-k”) insulation structure, source and drain electrodes, and a light emitting structure. The active layer is disposed on the substrate. The gate electrode is disposed on the active layer. The first high-k insulation structure is disposed on the gate electrode and includes a carbon-doped first high-k insulation layer and a first ammonia layer on the carbon-doped first high-k insulation layer. The source and drain electrodes are disposed on the first high-k insulation structure and constitute a semiconductor element together with the active layer and the gate electrode. The light emitting structure is disposed on the source and drain electrodes.