Recently, the applications of light emitting diodes (LED) in illumination are up roaring, and the solutions with high color rendition under various color temperature are in urgent need. Therefore, an effective method for improving lighting properties is very critical and necessary. To study new material of phosphor to combine with the LED chip or new LED structure to emit different spectrum could improve them. In addition, to combine with different kind of LEDs could achieve the same purpose and even with much lower cost. In this work, it has developed such an optimization method for a common white LED combined with a color LED. This method allows for finding the optimal mixing ratio of luminance of both LEDs in order to obtain the best CRI for illumination. By combining with a specific color LED, a white LED whose color temperature is approximately 6500K and CRI is 70, can be improved its color rendering index better than 80 and its correlated color temperature down to near 3000K simultaneously. Moreover, it could produce one which is more close to the warm-white lighting, and it is possible to substitute for the incandescent lamp. Thus, the mixing light is more suitable for illumination in housing applications. In the same way, with more LEDs in combination, CRI could be enhanced much better.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.