A sample of polyurethane ionomer is studied by small angle X-ray scattering, positron annihilation and density measurement. The experimental results demonstrate that the counter-ion radius, domain volume and density of polyurethane ionomer increase and free volume decreases with valency of the metal ion increasing. The application ranges of small angle X-ray scattering and positron annihilation are discussed and showed that positron annihilation is one of significant methods for studying domain volume in ionomer.