As potential color converter towards white/red light-emitting diodes, novel Ca2GdSbO6:Mn4+ phosphors with excellent optical performances were prepared by a conventional solid-state reaction route. The as-prepared phosphors with monoclinic crystal system had abundant [SbO6] octahedrons for Mn4+ ions to occupy and stably exist. With an excitation of 356 nm, an intense red emission peaking at 676 nm attributed to E-2(g)->(4)A(2g) transition of Mn4+ ions can be observed in the emission spectrum. The critical concentration of Mn4+ ions was found to be 0.6 mol% and the concentration quenching mechanism was also discussed in detail. Importantly, the Ca2GdSbO6:0.6%Mn4+ phosphors exhibited a high internal quantum efficiency of 38.9%.