A new route to synthesize amphiphilic core-shell particles that consist of well-defined hydrophobic polymer cores and poly(vinylamine) (PVAm) shells has been developed. The PVAm was treated with a small amount of tert-butyl hydroperoxide to generate free radicals that subsequently initiated both graft- and homo-polymerization of vinyl monomer such as n-butyl acrylate, methyl methacrylate, and styrene. Stable particles in the range from 100 to 250 nm in diameter with very narrow size distributions (polydispersity index between 1.08 and 1.15) were produced in high yields. TEM images of the particles revealed that they had well-defined core-shell nanostructures with thick and hairy PVAm shells. The structures of the vinyl monomer and water-soluble polymer were found to strongly influence the formation of particles and their sizes.