Flexible white OLED (organic light emitting diode) is a subject of intense research due to its application as versatile lightings for housing and flexible backlight unit for TFT-LCD. In order to make flexible OLED, flexible transparent conductor substrate is required which can replace ITO-glass used in current OLED devices. Another important point is a good gas barrier property of the flexible ITO-film, since the water vapor transmission rate (WVTR) of polymer film is much higher than that of ITO-glass and the life time of OLED device is very sensitive to the transmitted water vapor. In this work, a wet process of forming gas barrier layer on polyethersulfone (PES) film was studied utilizing spin coating of polysilazane solution and thermal conversion to thin SiO(2) barrier layer. The flexible white OLED fabricated with this ITO-SiO(2) (polysilazane)-PES substrate exhibited about equal power and current efficiencies to the OLED made with ITO-glass.