Nanostructure changes with the Krafft transition of complexes of poly(acrylicacid) with octadecyltrimethylammonium (PAA-OTA) in the aqueous solutions at various NaCl concentrations (Cs) from 20 to 400 mM, were studied making temperature-scanning small-angle X-ray scattering (SAXS) experiments and differential scanning calorimetric (DSC) measurements. For the PAA-OTA complex in the solution at a higher temperature than 25 degrees C, four SAXS peaks with a spacing ratio of 1:3(1/2):4(1/2):7(1/2), indicating the 2D hexagonal structure, were observed at Cs below 100 mM and two SAXS peaks with a spacing ratio of 1:2, indicating the lamella structure, were observed at Cs above 200 mM. For the complex in the solution at a lower temperature than 22 degrees C, a broad SAXS peak was observed at the scattering vector q = 1.2 nm(-1) when the Cs was less than 200 mM but not when Cs was 400 mM. Two peaks with a spacing ratio 1:2, indicating the lamella structure, were also observed for the complex in the solution at 8 degrees C. The DSC data demonstrated that the nanostructure changes were accompanied with the endothermic enthalpy change. On the basis of the experimental results, the salt concentration dependent nanostructures are discussed.