In this thesis, we study the optical model to describe the YAG powder as he yellow phosphor in white light LEDs with GaN chips. We apply Monte-Carlo ray tracing and volume Mie scattering to simulate the blue light and re-generated yellow light in the phosphor powder. We first record the position-dependent absorption of the blue lights and transfer into the yellow lights. Accordingly, we can describe the generation of white lights by the phosphor-based LED. Through the comparison between the experimental measurement and simulation, we successfully build up the optical model of YAG phosphors. Finally, we apply the model to the design of LED package for obtaining accurate light output and color behavior.