Thermally stable core-shell gold nanoparticles with highly-grafted polymer shells were synthesized by com-bining reversible addition-fragmentation transfer (RAFT) polymerization and click chemistry of copper-catalyzed azide-alkyne cycloaddition. (CuAAC) First, alkyne-terminated poly(styrene-b-4-vinylbenzyl chlo-ride) (Alkyne-PS-b-P4BC) was prepared from the alkyne-terminated RAFT agent. And azide functionalized gold nanoparticles (Au-$N_3$) were prepared by the functionalization of gold nanoparticles with 1-bromo-11-undecanethiol followed by nucleophilic substitution with sodium azide. The reaction was confirmed by NMR and FTIR. And then alkyne-PS-b-P4BC was coupled to Au-$N_3$ by CuAAC in THF in the presence of copper(II) sulfate and sodium ascorbate. Careful characterization using FTIR, UV-VIS, and TGA showed that PS chains were successfully grafted onto Au nanoparticle surface with high grafting density. To prove the importance of highly packed polymer shell on the nanoparticles, our PS coated Au nanoparticles (PS-Au1) were added into the PS and PS-b-P2VP matrix, respectively. For control sample, another PS-Au nano-particle was made by conventional “grafting to” approach (PS-Au2) and their behavior in PS-b-P2VP matrix was compared. TEM and UV-VIS analysis clearly showed that PS-Au1 nanoparticles were well dispersed in the PS matrix without any aggregation. Whereas the PS-Au2 nanoparticles were strongly segregated to the interface between PS and P2VP domains due to the favorable interaction between the P2VP matrix chains and the exposed Au bare surface, the PS-Au1 nanoparticles with high grafting density were mainly located in the PS domain. In addition, all PS-Au1 nanoparticles were found without any aggregation at annealing of 220 °C for 2 days. Our simple but powerful approach can be easily extend to many core-shell inorganic nanoparticles.