Polystyrene-block-poly(n-butyl methacrylate) copolymer (PS-b-PnBMA) exhibits only a lower disorder-to-order transition (LDOT), not a closed-loop phase behavior, within an experimentally accessible temperature range. But, when mixed solvents of di-n-octyl phthalate and hexadecane were added to PS-b-PnBMA, this mixture exhibited the upper order-to-disorder transition (UODT) below the degradation temperature as well as an LDOT above the glass transition temperatures of both blocks; thus, a close-loop phase behavior was observed. These results suggest that the closed-loop phase behavior may be a general phenomenon for weakly interacting polystyrene-block-poly(n-alkyl methacrylate) copolymers with LDOT.