Abstract: The monodispersed NaGdF4:Yb, Er nanoparticles were synthesized via a solvothermal approach. Rare-earth stearate was used as the precursor. The properties of as-prepared nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), fluorescence spectra and magnetic hysteresis loop (MHL). Experimental results showed that the nanoparticles are single crystal hexagonal phases with an average size of around 20 nm, exhibiting good superparamagnetic and excellent up-conversion fluorescent characteristics. The as-prepared nanoparticles were modified with amino groups by Stober method, and linked with transferrin to make it to be a probe. The probe was applied to the bio-labeling and imaging of HeLa cells. This method was proved to have good specificity, and the infrared light excitation can effectively avoid the autofluorescence of biological samples.