Upper critical solution temperature (UCST)-type phase separation behavior and its conformational change of well-defined poly(N-isopropylacrylamide) (pNIPAm) in deuterated room-temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (d(8)-[C(2)mIm(+)][TFSA(-)]), were investigated by means of dynamic light scattering (DLS) and small-angle neutron scattering (SANS) measurements. From the temperature dependence of time-averaged scattering intensity obtained by DLS, it was found that the cloud points of pNIPAm/d(8)-[C(2)mIm(+)][TFSA(-)] solutions increased with molecular weight (M-w) and concentration. In addition, it was found that there are two relaxation modes of pNIPAm in the IL solutions. From SANS measurements, the radius of gyration, R-g, and the Flory-Huggins interaction parameter, chi, were evaluated as a function of temperature during the phase separation.