In the present work, geometric structure, electronic properties, and 4f -> 5d transitions of gamma=Ca2SiO4:Ce3+ phosphors have been investigated by using first-principles calculations: Four categories of typical stbstitutioris (i.e., the doping of the Ce3+ without the neighboring dopants/defects and with the neighboring V-o(center dot center dot), Al-Si', and V-Ca '') are taken into account to simulate local environments of the Ce3+ located at two crystallographically different calcium sites in the gamma-Ca2SiO4. Density functional theory (DFT) geometry -optimization calculations are first performed on the constructed supercells to obtain the information about the local structures and preferred sites for the Ce3+. On the basis of the optimized crystal structures, the electronic properties of gamma-Ca2SiO4:Ce3+ phosphors are calculated with the Heyd-Scuseria-Ernzerhof screened hybrid functional, and the energies and relative oscillator strengths of the 4f -> 5d transitions of the Ce3+ are derived from the oh initio embedded cluster calculations at the CASSCF/CASPT2/RASSI-SO level. A. satisfactory agreement with the available experimental results is thus achieved. Moreover; the relationships between the dopants/defects and the electronic as well as spectroscopic properties of gamma-Ca2SiO4:Ce3+ phosphors have been explored. Such information is vital, not least for the design of Ce3+-based phosphors for the White light-emitting diodes (w-LEDs) with excellent performance.