The Structure of crystals and ionic aggregates have in poly(ethylene-co-methacrylic acid) ionomers cast from its aqueous solutions which were prepared the various degree of neutralization with K cation been studied. DSC thermograms showed only one endothermic peak at around 80°C was attributed to melting of crystalline parts and no peek of the crystallites formed isothermally by annealing at room temperature was observed. The temperature and intensity of the endothemic peak was scarcely changed by the degree of neutralization. This finding suggested that ionic aggregates have no effect on crystallization. The ionic cluster peak in SAXS profile was analyzed by the modified hard-sphere model proposed by Yarusso and Cooper. The size and distance of the ionic aggregates were increased with the degree of neutralization, while the number of the ionic aggregates was decreased. Particularly, the number of ionic aggregates in the samples cast from its aqueous solution is many in comparison with the samples prepared by the melting-mixture method.