Up-conversion nanoparticles made from NaYF4:Yb3+, Er3+ have been used as imaging materials in many studies. These stable luminescence materials always needs hydrophilic modification to meet the challenges in biomedical applications. In our work, hydrophilic up-conversion nanoparticles (HUNPs) with stable luminescence character were prepared by a one-step method. The excitation wavelength of this type of nanoparticles is 980 nm. Furthermore, from the data of zeta potential measurement and energy dispersive X-ray (EDX) analysis, we deduced that these nanoparticles exhibited amino groups on the surface. MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide) study showed that the as-prepared up-conversion nanoparticles demonstrated low cytotoxicity and excellent biocompatibility.