Mesomorphic phase behavior and phase diagram of mixtures of omega-lithium sulfonatopolystyrene (PSsul) with trimethylstearylammonium chloride (TMSAC) were investigated with differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). DSC measurements indicated that the PSsul and the TMSAC formed ionic aggregate with 70 wt % of PSsul and cooperatively crystallized in the aggregate. The SAXS measurements indicated the PSsul-TMSAC aggregate in solid state formed a hierarchical lamellar-within-lamellar structure which had two lamellae with different periodic lengths. Above melting temperature of the aggregate, the shorter period disappeared and the PSsul-TMSAC aggregate formed single lamellar structure. When weight fraction of PSsul was less than 0.7, the PSsul-TMSAC mixtures were phase-separated and had two types of mesomorphic structures attributed to TMSAC itself and the ionic PSsul-TMSAC aggregate. On the other hand, when the weight fraction of PSsul was higher than 0.7, the mesomorphic morphologies of the PSsul-TMSAC mixtures changed in order of lamellar, gyroid, and ordered sphere morphologies with increasing concentration of PSsul. The phase diagram summarizing these DSC and SAXS results was comprised of 12 regions.