A novel polypeptide, poly(O-phospho-L-Ser), was synthesized through the following three steps: i) preparation of O-diphenylphospho-L-serine N-carboxyanhydride [Ser(PPh2) NCA]; ii) polymerization of Ser(PPh2) NCA; iii) removal of phenyl groups from poly[Ser(PPh2)]. The overall yield in the three-step synthesis was 71%, and the phospho-Ser content in poly(O-phospho-L-Ser) was 98 +/- 1%. Circular dichroism spectra suggest that the polypeptide has two different disordered structures in aqueous solution.