In this study, an amphiphilic poly(ethylene glycol)methyl ether-block-poly(glycidyl methacrylate) diblock copolymer (MPEG-b-PGMA) was synthesized via atom transfer radical polymerization (ATRP), and its micellar solution was prepared in acetone/water mixture. Core cross-linked (CCL) micelles were synthesized by cross-linking the epoxy functional group of poly(glycidyl methacrylate) block with ethylenediamine. These CCL micelles were used in the synthesis and stabilization of platinum nanoparticles (PtNPs) in aqueous media. Transmission electron microscopy (TEM) images showed that well-dispersed PtNPs with a diameter of around 5 nm were formed within the MPEG-b-PGMA spherical CCL micelles having 22.0 +/- 3.0 nm diameters. The mean TEM diameter of the PtNPs is of the order of several nanometers, which is consistent with the plasmon absorption peaks observed at around 205 and 261 nm. The catalytic activity of CCL micelle-PtNP dispersion was also investigated in the reduction of p-nitrophenol to p-aminophenol in the presence of NaBH4. The results showed that the PtNPs exhibit a good catalytic activity toward reduction of p-nitrophenol. CCL micelle-stabilized PtNP dispersions were stable for long periods of time without changing properties at room temperature. CCL micelles found to be good hostage or stabilizer for the NPs in aqueous media.