Dual-modal in vivo tumor imaging and photodynamic therapy using hexagonal NaYF4:Yb,Er/NaGdF4 core-shell upconverting nanoparticles combined with a photosensitizer, chlorin e6, is reported. Tumors can be clearly observed not only in the upconversion luminescence image but also in the magnetic resonance image. In vivo photodynamic therapy by systemic administration is demonstrated under 980 nm irradiation.