Phosphor yttrium aluminum garnet (Y3Al5O12, YAG) doped with Eu (10 at%) was synthesized by supercritical water (SCW) and solid-state methods in a batch reactor. The crystals, morphologies and luminescent properties of the phosphors obtained from different pH conditions in the SCW were studied. Pure YAG phase was successfully synthesized at the alkaline condition in the SCW without formation of intermediate phases, while the size and luminescent property of YAG : Eu were strongly affected by pH condition. The luminescent intensity of YAG : Eu by the SCW method without further thermal treatment is stronger than that by the solid-state method with thermal treatment.