The dynamic mechanical properties of styrene-co-styrenesulfonate (SA) and styrene-co-methacrylate (MA) ionomers containing 30 wt% of fatty acid salts obtained from either soybean oil or lard oil were investigated. It was observed that when the fatty acid salts were added to the SA ionomers (2.7 or 6.3 mol% of ionic groups), the ionic modulus increased, but the cluster T (g) decreased strongly. In the case of the MA ionomer at 2.4 mol% of ion content, the addition of the fatty acid salts also increased the ionic modulus, but decreased the cluster T (g) slowly. On the other hand, for the MA ionomer at 6.9 mol% of ion content, the ionic modulus was not changed much by the addition of acid salts, but the cluster T (g) was decreased significantly. The above findings indicated that the roles of the fatty acid salts in the styrene-based ionomers were changed by the types and amounts of ionic groups of the ionomers. The SAXS results were also consistent with this conclusion. Finally, it was suggested that the fatty acid salts could be used as both eco-friendly plasticizer and filler for the ionomers.