In order to fabricate organic light emitting diode (OLED) on color filter layer for full color OLED, we estimated the property of water absorption of the overcoat (OC) layer and indium tin oxide (ITO) on the OC layer. During ITO patterning process, OC layer was exposed water and absorbed that. Due to property of water absorption of overcoat layer, devices on overcoat layer showed the low efficiency and short life time. Especially, in the device with overcoat, dark spot happened very seriously. To solve this problem, the heat treatment process was added to desorb water from OC layer and the SiO2 buffer layer was adopted as the blocking layer for water to sink through OC layer. Lifetime of device with suggested structure and process was improved dramatically over twenty times.