The potential of rare earth doped nanophosphors makes them appealing for a myriad of important applications such as white light emitting diodes (LEDs), displays, lasers, fluorescent labels, and solar cells. While the preparation of rare earth doped nanophosphors is a challenging task, it is technologically beneficial to produce nanoscale phosphors and to understand their size-dependent optical properties in order to support more opportunities for new applications. Currently, cerium-doped yttrium aluminum garnet (YAG: Ce3+) is the most complex oxide and its unique optical property makes it an ideal material for this study. However, there are problems in synthesizing the YAG: Ce3+ nanophosphors, especially the lacking in an approach to produce the nanophosphor with a size of less than 10 nm, and furthermore, not much is known about its size-dependent optical properties like the coupling strength of an electron-lattice relaxation. This study aims to synthesize highly crystalline YAG: Ce3+ nanophosphors with particle sizes targeted at less than 10 nm, 100 nm, and 300 nm and study the size effects on the optical properties of YAG: Ce3+.