Phase behavior of ionic liquid [HMIM][PF6] has been investigated under high pressure up to 5.6 GPa at room temperature. The results indicated that [HMIM][PF6] might experience a phase transition at about 3.4 GPa upon compression, which could be identified as solidification to superpressurized glass by ruby R-1 line broadening measurement and synchrotron X-ray diffraction (XRD) patterns. For conformational equilibrium, the fraction of all-anti (AAAA) conformer increased upon compression, while the conformational change was independent of temperature. These facts indicated that there were large differences of the structure in response to the extreme conditions, especially in the structure of the cation. (C) 2015 Elsevier B.V. All rights reserved.