An OLED device includes a substrate, a first active layer, a first gate electrode, a second gate electrode, first source and first drain electrodes, a first high dielectric constant (high-k) insulation structure, and a light emitting structure. The substrate has a first region and a second region. The first active layer is disposed in the first region on the substrate. The first gate electrode is disposed on the first active layer, and has a first thickness. The second gate electrode is disposed on the first gate electrode. The first source electrode and first drain electrode are disposed on the second gate electrode, and constitutes a first semiconductor element together with the first active layer and the first gate electrode. The first high-k insulation structure is disposed between the first gate electrode and the second gate electrode, and is spaced apart from the first source electrode and first drain electrode.