The heat capacity C p of 1-butyl-3-methylimidazolium chloride, bmimCl, which is a prototype ionic liquid, was measured with an adiabatic calorimeter in the temperature range 5–380K. A single crystal of bmimCl fused at 341K with excess entropy of 77.1JK −1 mol −1 . In the supercooled phase, a glass transition with a large C p jump occurred at 225K. The low temperature C p of glassy bmimCl showed a large low-energy excitation usually called boson peak. We concluded that ionic liquid bmimCl is quite disordered and fragile as glass-forming molecular liquids from a thermodynamic point of view.