Isotactic poly (N-isopropylacrylamides) (PNIPAMs) with a wide range of tacticity exhibit upper critical solution temperature (USCT) type phase transition in an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI]). The phase transition temperature becomes higher with increasing isotacticity, since more amide groups are aligned along the same side of backbones to form stronger intramolecular hydrogen bonding. Phase diagrams of highly isotactic PNIPAMs show prominent UCST peaks at low concentrations, similar with the typical polymer solution, which implies the dissolution of isotactic PNIPAMs is dominated by the entropy-driven effect. The increase in phase transition temperature with molecular weight is also from the entropic reason. The phenomena above reveal that the phase transition behavior of PNIPAMs in ionic liquids is highly influenced by tacticity effect.