Monodisperse cobalt nanoparticles with relatively high coercivities were synthesized by thermal decomposition of cobalt acetate for the application of ultra-high density magnetic recording media. Without using any reducing agents, Co nanoparticles of 13.9-21.3 nm in size were prepared using various protecting agents and a high boiling point solvent trioctylamine. The Co nanoparticles prepared with the mixture of oleic acid, oleylarnine, and polyvinylpyrrolidone (PVP) showed clear crystalline structure of face centered cubic, and their coercivity was measured to be 27.2 kA/m PVP was effective in controlling particle growth while hindering agglomeration between the particles. The as-prepared Co nanoparticles with a moderate coercivity can be applicable to patterned media.