A simple bilayer structure of organic light emitting diode (OLED) was used to study the characteristics of various anode preparations. Systematic investigation of indium tin oxide (ITO) anode surface treatment of OLEDs was performed to get the optimum condition for the ITO anode. The. ITO surface was treated by Ar or O-2 plasma with different RF power, chamber pressure and exposure time. Atomic force microscopy was used in measuring the surface morphology and roughness analyses. Sheet resistance of ITO anodes reduces to less than 7 Omega/square and surface roughness of ITO anode decreases more than 20%. Hence, the electric characteristics of OLED were improved. The operating voltage of OLED devices with Ar or O-2 plasma treated anodes decreased from 7.5 to 6.5 V and 6.0 V, respectively. We also show the presence of less dark spots in the OLEDs with plasma-treated ITO anode. As a result, the stability and lifetime of OLEDs can be improved obviously. (C) 2003 Elsevier B.V. All rights reserved.