We show that ${\mathrm{Ce}}^{3+}$ can be an efficient sensitizer for ${\mathrm{Yb}}^{3+}$ in the host lattice of yttrium aluminum garnet (YAG). With blue-light excitation to induce the $4f\to 5d$ transition of ${\mathrm{Ce}}^{3+}$ , characteristic near-IR emission of ${\mathrm{Yb}}^{3+}$ due to transition of ${}^{2}F_{5∕2}\to {}^{2}F_{7∕2}$ peaking at $1030\mathrm{nm}$ is generated as a result of energy transfer from ${\mathrm{Ce}}^{3+}$ to ${\mathrm{Yb}}^{3+}$ . The result of spectral evolution with temperature indicates that the efficiency of energy transfer is enhanced owing to thermal effect. This evidence implies that the phonon-assisted process participates in the downconversion of YAG: ${\mathrm{Ce}}^{3+}$ , ${\mathrm{Yb}}^{3+}$ .