In Ce:YAG–GC, Ce 3+ ions, incorporated in the Y sites of the precipitated Ce:YAG micro crystals, emit yellow fluorescent light when they are optically excited by a blue LED. White light emission can be obtained by mixing the exciting blue light and the fluorescent yellow light. In the emission process of Ce:YAG– GC, light scattering generally occurs at the phase boundary between the precipitated Ce:YAG micro crystals and the glassmatrix phase due to the difference in refractive index. Since the light scattering changes the direction of both the exciting blue light and the fluorescent yellow light, this directly affects the luminous efficacy of Ce:YAG–GC phosphor. In this study, the interdependence between Ce:YAG particle si ze, luminous efficacy, and the light scattering coefficient were investigated in order to improve the luminous efficacy of Ce:YAG–GC phosphor. Since the luminous efficacy should be affected by the light scattering, as mentioned above, an understanding of light scattering phenomena is critical for the development of Ce:YAG–GC phosphor with higher luminous efficacy. 2. Experimental