Melanin-like nanoparticles were synthesized with size control through neutralization of dopamine hydrochloride with NaOH, followed by spontaneous air oxidation of dopamine. Although the particle characteristic of natural melanins was understood to be significantly affected by the biological and structural environment, melanin-lke nanoparticles can be realized through the chemical reactions only. Melanin-like nanoparticles that are <100 nm showed excellent dispersion stability in water as well as biological media and good biocompatibility to HeLa cells after the appropriate surface modification with thiol-terminated methoxy-poly(ethylene glycol) (mPEG-SH). Furthermore, the demonstrated ability of melanin-like nanoparticles to reduce 2,2-diphenyl-1-picrylhydrazyl (DPPH) suggests free radical scavenging activity of the material.