In this paper,CMC/Eu complex nanoparticles were synthesized at different pH values by the reaction of Eu 3 + and biodegradable carboxymethyl cellulose (CMC). FT- IR results confirm that the carboxyl groups on CMCchains,the oxygen atoms of unsubstituted hydroxyl groups and the oxygen atoms of ether bonds all involve in thereaction with Eu 3 + .SEM and TEM results show that the synthesized CMC/Eu particles are in a nanoscale andpresent weak polycrystalline structure.Solid UV- Vis absorption spectra show that CMC is a good energy donor andfluorescent sensitizer.Moreover,it is found that there is a strong dependence of particle size and distribution on thefluorescence properties of CMC/Eu nanoparticles,and that pH value greatly affects the particle size,the particlesize distribution and the fluorescence properties; for instance,the sample synthesized at pH 7 is of the smallest par-ticle size (150 ～200nm),the most uniform size distribution,the strongest fluorescence intensity and high fluores-cence monochromaticity,and,with the increase of pH value,the nanoparticles aggregate more seriously,the particlesize increases,and the fluorescence intensity accordingly reduces.All these findings reflect the quantum size effect.