The effect of ionic interaction on linear and nonlinear viscoelastic properties was investigated using poly(ethylene-co-methacrylic acid) (E/MAA) and its ionomers which were partially neutralized by zinc or sodium. Dynamic shear viscosity and step-shear stress relaxation studies were performed. Stress relaxation moduli G(t, gamma) of the E/MAA and its sodium or zinc ionomers were factorized into linear relaxation moduli G degrees(t) and damping functions h(gamma). The relaxation modulus at the smallest strain in each ionomer agreed with the linear relaxation modulus calculated from storage modulus G＇ and loss modulus G ＇＇. In the linear region, the ionic interaction shifted the relaxation time longer with keeping the same relaxation time distribution as E/MAA. In the nonlinear region, the ionic interaction had no influence on h(gamma) when the ion content was low. At higher ion content, however, the ion bonding enhanced the strain softening of h(gamma).