Organic light emitting diode (OLED) has high contrast, high response and electrical power saving ability. The emission spectrum from OLED structure depends on film thickness and refractive index of the thin films. In addition, the designing process becomes more difficult increasing the number of materials used for OLED. The structural optimization of thin films is useful to satisfy a design specification. We developed the system which could optimize film thickness and refractive index of each layer simultaneously by genetic algorithm. In this paper, we report results of the optimum design of OLED by the genetic algorithm and the two-stage optimization using the conjugate gradient method.