Water-soluble NaGdF4 : Eu fluorescent nanoparticles modified by citrate were synthesized by hydrothermal method with stable fluorescent properties. It was found that the fluorescence of the solution of as-prepared particles could be quenched by Cu2+, and thus a new mathod to determine trace Cu2+ using NaGdF4 : Eu as fluorescent probe was established. A pH 10.0 and the concentration 1.0 x 10(-3) mol x L(-1) of NaGdF4 : Eu were selected for measurement Besides, the effect of some foreign ions on the fluorescence signals was investigated and the interference of Fe3+ was found, which was eliminated by adding triethanolamine. The regression equation of standard curve was I = 532-0.685c with the correlation coefficient of -0.998 4 when the concentration of Cu2+ was in the range of 3.33 x 10(-6) -1.33 x 10(-4) mol x L(-1), and the detection limit of 8.9 x 10(-7) mol x L(-1) and a RSD of 0.62% for 11 replicates of a 6.0 x 10(-5) mol x L(-1) Cu2+ solution were obtained, which suggest a wide linear analytical range, high sensitivity and high precision. Analytical applicability of the particles was demonstrated by tea sample analysis and the results of Cu2+ determination were in good agreement with those obtained by atomic absorption spectrometry. The reason for fluorescence quenching by Cu2+ can be explained in terms of combination of Cu2+ with citrate on the surface of NaGdF4 : Eu particles leading to a change in surface structure and the composition.