Relationships between surface state of indium tin oxide (ITO) and luminescent properties, and aging characteristics of organic light emitting diodes (OLEDs) were investigated. Polycrystalline ITO surface used as the substrate of the thin film OLED was changed by chemical etching and ultraviolet (UV) irradiation. Initial properties of luminescent characteristics showed no significant difference between different ITO surface states. On the other hand, the device fabricated on UV irradiated ITO showed longer lifetime than that of without UV irradiation. We found that UV irradiation was effective for the aging characteristics of thin film OLED. (c) 2004 Elsevier B.V. All rights reserved.