Early diagnosis that combines the high-resolutional CT and sensitive NIR-fluorescence bioimaging couldprovide more accurate information for cancerous tissues, which, however, remain a big challenge. Herewe report a simple bimodal imaging platform based on PEGylated NaYbF4: Tm3t nanopaticles (NPs) ofless than 20 nm in diameter for both CT and NIR-fluorescence bioimaging. The as-designed nanoprobesshowed excellent in vitro and in vivo performances in the dual-bioimaging, very low cytotoxicity and nodetectable tissue damge in one month. Remarkably, the Yb3t in the lattice of NaYbF4: Tm3t NPs functionsnot only as a promising CT contrast medium due to its high X-ray absorption coefficiency, but also anexcellent sensitizer contributing to the strong NIR-fluorescent emissions for its large NIR absorptioncross-section. In addition, these NPs could be easily excreted mainly via feces without detectableremnant in the animal bodies.