Heat capacities of liquid, stable crystal, and liquid-quenched glass of a room-temperature ionic liquid ( RTIL), 1-hexyl-3-methylimidazolium bis( trifluromethylsulfonyl) imide were measured between 5 and 310 K by adiabatic calorimetry. Heat capacity of the liquid at 298.15 K was determined for an IUPAC project as ( 631.6 +/- 0.5) J K-1 mol(-1). Fusion was observed at T-fus = 272.10 K for the stable crystalline phase, with enthalpy and entropy of fusion of 28.34 kJ mol-1 and 104.2 J K-1 mol(-1), respectively. The purity of the sample was estimated as 99.83 mol % by the fractional melting method. The liquid could be supercooled easily and the glass transition was observed around T-g approximate to 183 K, which was in agreement with the empirical relation, T-g approximate to ( 2/3) T-fus. The heat capacity of the liquid-quenched glass was larger than that of the crystal as a whole. In the lowest temperature region, however, the difference between the two showed a maximum around 6 K and a minimum around 15 K, at which the heat capacity of the glass was a little smaller than that of crystal.