Sodium-ion intercalation into the bulk of graphite hardly occurs. In this study, a nano-sized graphitizable carbon nanospheres (CNS) covered with basal planes were used to shorten the diffusion length, minimize side reaction and evaluate the dependence of graphitization degree. As a result, electrolyte decomposition was minimized by basal planes covering the surface and sodium-ion more clearly intercalating into CNSs than into ordinal graphite. It was also clarified that sodium-ion inserted into the bulk of CNSs as the graphitization degree decreased.