By using a two-stage soapless emulsion polymerization, four kinds of core-shell nanoparticles have been prepared,which are composed of a polystyrene core having an average diameter of 256 nm and a poly(methyl methacrylate-co-acrylic acid) shell. The transmission electron microscopy (TEM) micrographs and the atomic force microscopy (AFM) images evidenced the presence of a core-shell structure. In the infrared spectra, the shift of VCOOH to lower wavenumber with increasing Eu3+ ion content indicates that coordination between the oxygen of the carboxylic group and Eu3+ has occurred.The fluorescence intensity of 5D0-7F2 transition was observed to reach its maximum with a carboxyl group molar percentage of 40% in the shell and an Eu3+/-COO- molar ratio of 1:3. The fluorescence intensity ratio of 5D0-7F2 to 5D0-7F1 transition reached its maximum with an Eu3+/-COO- molar ratio of 1:3 for all the four series.