This paper addresses the carbon co-doping of Ce-doped garnets, an efficient scintillation material and white light phosphor. The composition and the optical and scintillation properties of carbon-co-doped Y3Al5O12 (YAG) are studied at different cerium and carbon concentrations. YAG:Ce,C crystals were grown in the Ar + CO atmosphere from W crucibles. The carbon concentration reaches 0.5 atom %, but it does not affect the Ce distribution coefficient. The appearance and elimination of color centers are discussed in comparison to Ce-free YAG:C crystals. The tuning of cerium and carbon concentrations provides a light yield enhancement of up to 29 »600 phot/MeV. The achieved enhancement may extend the application range of garnet-type phosphors and scintillators.