Two kinds of linear and star thermo-responsive polymers of poly(N-acryloylsarcosine methyl ester) [(PNASME)n] and poly(N-isopropylacrylamide) [(PNIPAM)n] with the arm number n = 1, 2, 3 and 4 were synthesized following the core-first method via solution RAFT polymerization employing a series of mono- and multi-functional chain transfer agents (CTAs). It is found that the phase transition of the two thermo-responsive star polymers of (PNASME)n and (PNIPAM)n is firmly correlative to the polymer topology. That is, linear polymer has a higher phase transition temperature (PTT) than star polymers and the PTT of star polymers generally decreases with the increasing arm number n. Star (PNASME)n is more dehydrated at temperature above PTT than linear one. The possible reasons leading to the lower PTT and higher dehydrated degree of star polymers than linear ones are discussed, and the compact conformation of star polymers is ascribed.