Though L-arginine-containing polymers show versatile biological functions, a precisely controlled synthesis of poly(ethylene glycol)-b-poly(L-arginine) (PEG-b-PArg) block copolymers has not been reported. Here, an effective method for the synthesis of PEG-b-PArg block copolymers is developed. In order to obtain PEG-b-PArg, a two-step reaction, i.e., synthesis of PEG-b-poly(L-ornithine) is employed, followed by guanidinylation with N,N'-bis(tert-butoxycarbonyl)-1H-pyrazole-1-carboxamidine. This procedure quantitatively converts amino groups to guanidium groups at the side chains of peptide segments under mild conditions. Polyion complex (PIC) micelles are prepared by mixing the positively charged PEG-b-PArg with negatively charged homo-polyelectrolytes such as hyaluronic acid (HA) or chondroitin sulfate C (CS). PIC micelles prepared with CS show a higher stability than those prepared with HA, probably due to strong interactions between guanidium cations in PEG-b-PArg and carboxylate/sulfate in CS. Thus, PIC micelles containing PArg are a potentially effective arginine carrier for the development of in vivo therapeutic applications for various diseases related to nitric oxide, which is generated from inducible nitric oxide synthase in macrophages using L-arginine as a substrate.