The detailed structure and unique characteristics of a sodium poly(4-styrenesulfonate)-grafted nanoparticle, CCL-SBS₁₁₃-b-SSNa₂₀₈, were investigated, where CCL, SBS, and SSNa represent a core-cross-linked micelle, 4-(1-methylsilacyclobutyl)styrene, and sodium 4-styrenesulfonate, respectively. The particle nanostructure was analyzed by small-angle neutron scattering (SANS), dynamic light scattering (DLS), and atomic force microscopy. SANS data suggested that the particle had a core–corona structure with a 14-nm core radius (R_c) and 2-nm radius of gyration (R_g) corona-forming chain in water. DLS analysis revealed that the hydrodynamic radius (R_h) of particle was 100 nm in water, which is much larger than the whole particle size evaluated by SANS, but the R_h value gradually decreased with addition of NaCl and reached a constant value of 61 nm at an NaCl concentration above 0.2 M. Conductometric titration of the acidic form sample with NaOH_aq suggested that the CCL-micelle had exactly the same content of sulfonate groups as the precursor block copolymer. The polySSNa-grafting particle showed high solution stability toward salt addition, which may be due to the electrostatic stabilization effect in addition to steric stabilization by the grafting polymer chains.