Well-defined cylindrical polymer brushes with poly(acrylic acid) (PAA) cores and poly(n-butyl acrylate) (PnBA) shells have been synthesized via a combination of anionic polymerization and atom transfer radical polymerization. These amphiphilic brushes are unimolecular cylindrical micelles and can be used as single-molecular templates for the synthesis of inorganic nanoparticles, because the carboxylic acid groups (or carboxylate groups, after neutralization) in the polymer core can coordinate with various metal ions such as Fe3+ and Fe2+. Ultrafine magnetic nanoparticles were successfully synthesized within the core of these polymer brushes, as confirmed by various characterization techniques. Mossbauer spectroscopy and magnetization measurements indicate that the hybrid nanocylinders exhibit interesting superparamagnetic behavior.