The down conversion of blue to white light with yttrium aluminum garnet (YAG) phosphor is analyzed theoretically for GaN/InGaN light emitting diodes. A cerium-doped YAG phosphor with particle size of having peak emission wavelength of 560 nm is considered in this study. Effects of phosphor concentration, thickness of the conversion medium, excitation spectrum, and driving current are studied in terms of luminous efficacy and the quality of white light emission. It has been observed that the above parameters have a significant effect on chromaticity coordinates. Ray-tracing simulation results show that the luminous efficacy of down-converted white light is found to be 3.25 times the blue light excitation with phosphor concentration of and thickness of 30 μm for an injection current density of . A stable cool white light having correlated color temperature in a range of 5500–5600 K is achieved for the proposed optimized design for the variation of ambient temperatures from to 150°C.