Abstract: The surface of ZnS quantum dots was modified with polyacrylic acid(PAA) by situ polymerization. The resulting ZnS@PAA composite nanoparticles were characterized by XRD, TEM, FTIR, TGA and fluorescence test. XRD results indicat that the ZnS@PAA composite nanoparticles exhibite a pure cubic phase. FTIR and TGA results reveal that the ZnS nano particles are coated with PAA. TEM results reveal that PAA coated ZnS quantum dots monodisperse and spherical shape in the water phase, the size of spherical particles is estimated to be 28 nm. The fluorescence tests show that after PAA coated, the fluorescent position and intensity of ZnS@PAA composite nanoparticles do not changed significantly. The test results display that PAA coated ZnS quantum dots have more favorable dispersion stability than that of the uncoated ones in aqueous system. The ZnS@PAA composite nanoparticles with excellen oxidation resistance, fluorescence stability, water-solubility and carboxyl-functionalization could be used for biological detection and bio-imaging.