The aggregation behavior of diblock copolymers with lower critical micellization temperatures (LCMT) in an ionic liquid (IL) is presented for the first time. Poly(methyl methacrylate) (PMMA), poly(benzyl methacrylate) (PBnMA), and polystyrene (PSt) are known to be soluble, low-temperature-soluble and high-temperature-insoluble, and insoluble, respectively, in a typical hydrophobic IL, 1-ethyl-3-methylimidazoliumbis(trifluoromethane sulfonyl)imide. Well-defined thermosensitive diblock copolymers, consisting of PMMA as the first segment and PBnMA as the second, with different molecular weights and compositions were successfully prepared by atom transfer radical polymerization to yield P(BnMA(2.2)-b-MMA(19)), P(BnMA(7.3)-b-MMA(19)), and P(BnMA(28)-b-BnMA(41)), where the numerals represent the number-average molecular weight of each segment (M-n/kDa). For comparison purposes, a thermo-insensitive diblock copolymer, consisting of PMMA and PSt (P(St(1.7)-b-MMA(19))), was also prepared. The LCMT thermosensitivity and self-assembled behavior of the diblock copolymers in the IL were investigated in terms of the scattering intensity and hydrodynamic radius of the particles using the dynamic light scattering technique.