||In recent years, there has been significant research on organic light emitting devices (OLED) due to their great potential for flat panel displays. Generally, their research mainly focuses on three aspects: the color, power luminous efficiency and lifetime of the OLED. To compete the performance of conventional LEDs, the power luminance efficiency is the most important issue. The calculation of the efficiency of OLED is under direct current (DC) driving. However, for flat panel display applications such as a passive matrix OLED, the pixels are driven by pulse voltage and under a time division multiplexing driving scheme. Under this condition, it is reported that the power luminous efficiency will be drop dramatically when increasing the number of scanning lines. The relationship between the drop of power luminous efficiency and number of scanning line will be discussed. Also, a convert from the power luminous efficiency which measured under DC driving to the power luminous efficiency which measured under time division multiplexing driving. In general, the surface emission angular pattern of OLED is assumed as a Lambertian radiation pattern. That is a surface emission angular pattern of cosθ. For calculating the power luminous efficiency η (η = L/P) with this assumption, the normal direction luminous of OLED can be converted to the total luminous flux L in the half space. However, it is reported that the surface emission angular pattern of the OLED is thickness dependent. The relationship between the thickness of OLED Alq layer and the surface emission angular pattern of the OLED will be discussed. Also, the modification of power luminous efficiency as a function of Alq thickness has been proposed.