Poly{2,5-bis[(4-butoxyphenyl)-oxycarbonyl]styrene} (PBPCS) with a re-entrant phase behavior is used to synthesize a new kind of ABC triblock copolymers (triBCPs), poly. Two well-defi ned triBCPs D 120 S 100 B 50 and D 120 S 100 B 120 with different molecular weights (MWs) and narrow MW distributions are synthesized by sequential atom transfer radical polymerization. The volume fractions of PDMS, PS, and PBPCS in D 120 S 100 B 50 are 0.24, 0.26, and 0.50, respectively, while the values in D 120 S 100 B 120 are 0.14, 0.16, and 0.70, respectively. At low temperatures, PBPCS is amorphous. Both samples form a core-shell hexagonally packed cylindrical (CSH) morphology. At high temperatures, PBPCS in the triBCPs can form a liquid crystalline (LC) phase, the transformation of which can induce an order−order transition (OOT) of D 120 S 100 B 120 . The nanostructure changes from a CSH structure to a three-phase-four-layer lamellar structure, and the transition is thermoreversible. The sequence of the OOT is opposite to that of a coil−coil−coil triBCP. The unusual LC behavior of PBPCS plays an important role on the thermoreversible morphological transition of the triBCP.