Abstract: In order to find a better thermal storage medium with a wide temperature range of liquid state, low melting point and good adaptation to industrial application, the thermal storage density of C₈mimPF₆ is calculated and compared with that of conventional thermal storage media. The results show that under the same conditions the thermal storage capacity of C₈mimPF₆ is over 30% higher than that of Thermex and about 47% higher than Neo SK 260. Although it is near 1/3 lower than that of the mixture of KNO₃, NaNO₃ and NaNO₂, because of its low melting point (-75 ℃), wide temperature range of liquid state (-75~416 ℃), almost no vapour pressure, high thermal and chemical stability, C₈mimPF₆ is indeed an ideal thermal storage medium.