The phase behavior of symmetric polystyrene-block-poly(n-butyl-ran-n-hexyl) methacrylate copolymers (PS-b-Pn(B-r-H)MA) with various molecular weights was investigated by small-angle X-ray scattering (SAXS), rheometry, polarized optical microscopy (POM), and static birefringence. PS-b-PnBMA exhibited a lower disorder-to-order transition (LDOT), whereas PS-b-PnHMA exhibited an order-to-disorder transition (UODT). However, when a random copolymer of Pn(B-r-H)MA was used as one of the blocks, PS-b-Pn(B-r-H)MA showed closed-loop phase behavior having both LDOT and UODT, which was seen in PS-b-poly(n-pentyl methacrylate) copolymer, when the total molecular weight was judiciously controlled. The phase behavior change by including a random copolymer was explained by an argument based on a compressible random phase approximation. We also found that the pressure coefficient of both transition temperatures of PS-b-Pn(B-T -H)MA was much larger than that of neat PS-b-PnBMA and PS-b-PnHMA.