We report, for the first time, the synthesis of the Y3Al5O12:Ce(3+) hollow phosphor particles with a uniform size distribution via the Kirkendall effect, characterized by using a combination of in situ X-ray diffraction and high-resolution transmission electron microscopy analyses as a function of calcination temperature. The formation of hollow Y3Al5O12:Ce(3+) particles was revealed to originate from the different diffusivities of atoms (Al and Y) in a diffusion couple, causing a supersaturation of lattice vacancies. The optical characterization using photoluminescence spectroscopy and scanning confocal microscopy clearly showed the evidence of YAG (yttrium aluminum garnet) hollow shells with emission at 545 nm. Another advantage of this methodology is that the size of hollow shells can be tunable by changing the size of initial nanotemplates that are spherical aluminum hydroxide nanoparticles. In this study, we synthesized the hollow shell particles with average diameters of 140 and 600 nm as representatives to show the range of particle sizes. Because of the unique structural and optical properties, the Y3Al5O12:Ce(3+) hollow shells can be another alternative to luminescence materials such as quantum dots and organic dyes, which promote their utilization in various fields, including optoelectronic and nanobio devices.