The pH-induced conformational transition of poly(sodium ethacrylate) PNaEA in aqueous solution, which occurs between a compact form at low charge-density and an extended coil at high charge-density, was studied by small-angle X-ray scattering and the structure at an each conformational state was analyzed and compared with the corresponding one of poly(sodium methacrylate) PNaMA. The conformational transition for PNaEA induced a remarkable change in the scattering data plotted in the form of the Kratky plot. By comparing the scattering data with theoretical scattering functions, it was clarified that the structures of the compact form and the extended coil are well mimicked by a swollen gel having a network structure and by a wormlike chain, respectively. Although such a structure of the extended coil of PNaEA is similar to the corresponding one of PNaMA, the structure of the compact form of PNaEA is different from the corresponding one of PNaMA, which is still represented by a wormlike chain in a Theta medium.