Upconverting nanoparticles (UCNPs) with fascinating properties hold great potential as nanotransducers for solving the problems that traditional photodynamic therapy (PDT) has been facing. In this report, by using well-selected bifunctional gadolinium (Gd)-ion-doped UCNPs and water-soluble methylene blue (MB) combined with the water-in-oil reverse microemulsion technique, we have succeeded in developing a new kind of UCNP/MB-based PDT drug, NaYF(4):Er/Yb/Gd@SiO(2)(MB), with a particle diameter less than 50 nm. Great efforts have been made to investigate the drug-formation mechanism and provide detailed physical and photochemical characterizations and the potential structure optimization of the as-designed PDT drug. We envision that such a PDT drug will become a potential theranostic nanomedicine for future near-infrared laser-triggered photodynamic therapy and simultaneous magnetic/optical bimodal imaging.