We investigated the effect of atmospheric-pressure plasma (APP) and low-pressure plasma (LPP) treatments on the performance of organic light emitting diodes (OLEDs) with an indium tin oxide (ITO) layer. The Owens-Wendt and Lifshitz-van der Waals acid-base methods revealed that the increase of surface energy was mainly attributed to polar component (gamma(p)(s)) and Lewis base (gamma(-)(s)) interactions, respectively, independent of either APP or LPP treatments. Unlike APP treatment, LPP treatment more plentifully produced reactive oxygen species in the plasma. Therefore, the LPP-treated ITO surfaces slowly proceeded with reorientation compared to APP-treated ITO. The carbon content in untreated ITO was approximately 0.045%, while those of Ar APP-, Ar/O-2 APP-, and O-2 LPP-treated ITO were 0.014, 0.011, and 0.010%, respectively, mostly containing incorporated reactive oxygen. The O-2 LPP-treated surface showed more uniform roughness than Ar or Ar/O-2 APP-treated surfaces. The highest work-function (Phi) value (4.58 eV) was obtained from O-2 LPP-treated ITO, intermediate values (4.47-4.48 eV) from Ar and Ar/O-2 APP-treated ITOs, and the smallest value from untreated ITO (4.46 eV). Thus, OLED fabricated on the surface of O-2 LPP-treated ITO substrate exhibited superior performance among all plasma-treated samples. (C) 2008 The Electrochemical Society.