Well-defined triblock copolymer nanospheres of polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene (PS-b-PNIPAM-b-PS) containing a hydrophobic core of the polystyrene (PS) block and a looped corona of the thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) block were prepared by the bifunctional macro-RAFT agent mediated dispersion polymerization via polymerization-induced self-assembly. This synthesis affords the fine control on the chemical composition of the PS-b-PNIPAM-b-PS triblock copolymer and on the diameter of the triblock copolymer nanospheres. By precisely investigating the thermoresponse of the PS-b-PNIPAM-b-PS triblock copolymer nanospheres and the poly(N-isopropylacrylamide)-block-polystyrene (PNIPAM-b-PS) diblock copolymer nanospheres under the strictly similar conditions, it is found that the phase transition of the looped PNIPAM chains takes place at a lower temperature and within a narrower temperature range than that of the linear PNIPAM chains. We believe that the present study is helpful to clarify how the topology of the tethered PNIPAM chains affects the phase transition at the lower critical solution temperature (LCST).