Polycaprolactone containing carboxyl groups (CPCL, (M) over bar(n) = 1200) was prepared from caprolactone (CL) and dimethylol propionic acid (DMPA). Based on the CPCL,4,4,'-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD), a novel series of polyurethane ionomers could be synthesized with ions in the soft segment. The tensile modulus and ultimate elongation of the ionomer cast films increased, and the glass transition temperature of the CPCL-rich phase moved to a higher temperature after ionization. Moreover, this kind of ionomers showed small amounts of hard seg ment crystallinity by DSC. WAXD shows an ionic peak at 2 theta = 8.6 degrees for the ionomer with carboxylate in the hard segment, which is not present in the ionomer with carboxylate in the soft segment.