In this work, we have examined modifications of structural and optical properties of Y3Al5O15:Ce3+ (Ce3+:YAG) material when transforming from nanograins to nanoceramic form under high pressure. Initial YAG material with 0.5% of Ce3+ was prepared using modified Pechini and transformed to nanoceramic at low temperature with a working pressure of 4 GPa. Structural properties of both samples (nanopowder and nanoceramic) have been examined with X-ray diffraction and scanning electron microscope. Optical properties have been examined using photoluminescence (PL) spectroscopy, PL-excitation spectroscopy at different temperatures, and time-resolved spectroscopy. Based on results obtained, excitation mechanism of Ce3+ ions has been explained. The main 560 nm emission band along with a new 400 nm emission band allowed to cover the whole visible spectral range. Moreover, it has been found that under high-pressure treatment the amorphous shell of YAG core appears, leading to increased 5d electrons–lattice interaction. This, in turn, reduces PL decay time and increases emission intensity of nanoceramic sample.