SrOx thin films were prepared by the ionization-assisted deposition as the electron injection layer between the electron transport/emitting layer and the Al cathode of the organic light emitting diode, By inserting an SrOx layer of a few-nm thick, the luminance increased nearly three orders of magnitudes for the same driving voltage. The device performance was also controlled by the ion acceleration voltage that was applied in the course of SrOx film deposition. Compared to the simple evaporation. the ionization-assisted deposition improved the luminance and the power efficiency by more than two times. XPS analysis suggested that the ion acceleration leads to two dimensional homogeneous film growth at the initial stage of film deposition.