Abstract: In order to prepare fluorescent labels which are to be applied to biological analysis, the water-soluble NaGdF4:Eu fluorescent nanoparticles were synthesized by hydrothermal method using gadolinium nitrate, europium nitrate, sodium fluoride and sodium citrate as starting materials. During the synthesis the temperature was controlled at 140°C for 4 hr with the pH value of the reacting solution 8.0. It was found that the fluorescent lifetime of NaGdF4:Eu colloids is 1.144 ms and the fluorescent intensity is high and stable. The synthesized particles were characterized and the IR spectrum shows that the as-prepared particles are coated with citrate. XRD patterns indicate that the synthesized particles are NaGdF4 crystals in hexagonal system and the solid solution is formed via the dopant Eu³⁺ entering into the NaGdF4 crystal lattice instead of Gd³⁺, of which the crystal size is about 11 nm. Moreover, the TEM image indicates a good dispersion of the synthesized particles which link the NaGdF4:Eu colloids with tripsin without the change in fluorescent spectra of NaGdF4:Eu colloids, though the particles cause the fluorescence of tripsin to be quenched. The quenching mechanism is discussed.