The phase behavior for the binary mixtures of a polystyrene-block-poly(n-pentyl methacrylate) (PS-b-PnPMA) and homopolystyrenes (PS) with various molecular weights was investigated by depolarized light scattering and small-angle X-ray scattering (SAXS) as a function of PS amount. A homopolymer-free PS-b-PnPMA, prepared by a high performance liquid chromatography (HPLC) technique, showed the entirely disordered (or homogeneous) state within experimental temperature range from the glass transition temperature (T-g) to the thermal degradation temperature (T-d). By adding PS to this PS-b-PnPMA, the closed-loop phase behavior was observed for the binary mixtures where no macrophase separation occurred. With increasing molecular weight (chain length) of PS, the ordered range of the closed-loop extended remarkably and a critical amount of PS increased, resulting in the conservation of only lamellar morphology Lip to the overall PS volume fraction (f(PS)) of 0.70 for a PS, having a chain length ratio (alpha) of 2 with respect to the PS chain length of PS-b-PnPMA.