The luminescence and magnetic resonance imaging (MRI) and photodynamic therapy (PDT) using lanthanide-doped upconverting nanoparticles (UCNPs) is reported. With the aim to induce a therapeutic effect, a photosensitizer, chlorin e6 (Ce6), was conjugated to UCNPs. Owing to the enhanced permeability and retention effect, UCNPs combined with a photosensitizer, chlorin e6 (UCNP-Ce6) could be readily accumulated at the tumors, which could be clearly observed not only in the upconversion luminescence image but also in the MRI image. Using the energy transfer from UCNPs to Ce6, cytotoxic singlet oxygen could be generated. An in vivo PDT effect by systemic administration of UCNP-Ce6 was demonstrated under 980-nm irradiation. These results indicate that UCNP-Ce6 can be used not only as dual-modal imaging probes for accurate diagnosis but also as photodynamic therapy agent for efficient therapy.