Ionic liquids can act as plasticizers possessing high-temperature stability for PMMA because of its low vapor pressure, thermal and chemical stability. PMMA/IL system can be used as gel electrolytes for batteries. As we all known, thermodynamic properties of polymers (enthalpy, volume etc.) will evolve slowly towards equilibrium below Tg, namely physical aging. Physical aging limits the stability of these materials. Characterization of physical aging is of essential importance for polymer from the point of both fundamental and applications. In this study, we use DSC to study glass-enthalpy relaxation of PMMA and PMMA/IL system during physical aging at 70 oC (below Tg). DSC curve shows that an enthalpy relaxation peak occurs in the vicinity of the glass-transition temperature after physical aging. As the time of aging increases, the peak increases and shifts to higher temperatures for both PMMA and PMMA/IL system because of the increase in the relaxation time with aging. At the same aging time, a larger peak can be observed for PMMA/IL system when compared with pure PMMA, which means the extent of physical aging is increased for PMMA/IL system. It can be concluded that the rate of physical aging is accelerated when adding IL into PMMA at the same aging temperature (Ta). This phenomenon is ascribed to two reasons. Firstly, IL as plasticizers weaken intermolecular forces and increase the intermolecular distance, so the molecular mobility is increased. Secondly, the Tg of PMMA is reduced when incorporated with IL, so the gap of PMMA/IL system between Ta and Tg is smaller than pure PMMA. The closer to Tg is for Ta, the faster the rate of physical aging is.