A novel synthetic route to peptide-capped gold nanoparticles was demonstrated herein. Tetrachloroaurate ions were reduced with 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) under extremely mild conditions (pH 7.2, ambient temperature) in the presence of cysteine-terminal desired peptides, so that peptide-capped spherical nanoparticles were successfully synthesized. Model basic peptides containing the Arg-Pro-Thr-Arg sequence, which is an essential motif that specifically binds to film surfaces composed of isotactic poly(methyl methacrylate), were employed. Particle sizes were approximately 10 nm, and size distributions were narrow. Positive zeta potentials of nanoparticles suggested the presence of the Arg-Pro-Thr-Arg sequence on the outermost surface. Thermogravimetric analysis revealed that peptides were closely packed on the gold's Surface. Parameters affecting reaction rates such as peptide structures and concentrations were investigated. Native peptide functions were conserved on nanoparticles by introducing a certain spacer between cysteine and the Arg-Pro-Thr-Arg sequence, suggesting that designing suitable peptide structures is essential to conserve peptide functions.