The effect of the imidazolium based ionic liquid (IL) 1-benzyl-3-methylimidazolium tetrafluoroborate [Bzmim][BF4]) was investigated on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution by using fluorescence, viscometric, and dynamic light scattering (DLS) techniques. The measurements were performed at four different [Bmim][BF4] concentrations (1-4 mg/mL) in PNIPAM aqueous solution. Our experimental results elucidate that the IL induces the collapsed globular structure of polymer, facilitated by the weakening of hydrogen bonds between the amide group of the polymer and water molecules; there; Ore, IL destabilizes the hydrated macromolecule structure. We observed that the phase transition of PNIPAM aqueous solution abruptly shifts down with increasing IL concentration mainly due to hydrophobic collapse and aggregation of a macromolecule. These results unambiguously reveal that the imidazolium based IL significantly affected the phase transition of PNIPAM and ruptured the hydrogen bonding between polymer and water molecules, and eventually the hydrated macromolecule structure was destabilized.