The phase behavior of the ionic liquid is supposed to depend on the thermal history. However, the details are not clear. The ionic liquid, 1-methyl-3-octylimidazolium tetrafluoroborate, abbreviated as [C₈mim]BF₄, has been known as a good glass-former, which can be cooled or heated at normal scanning rates without any phase transition. By heating from 183 K just below the glass transition temperature to 223 K, T_g + 33 K, a phase transition from the supercooled liquid to a liquid crystalline phase was observed by X-ray diffractometry. The assignment of the Bragg peaks indicates that a smectic A phase with a bilayer structure as seen in typical lamellar phases. The dielectric measurement was also performed on this material to investigate heating process depending on the thermal history. As the parameters of the thermal history, the initial temperature, T_ini, for heating measurement and the equilibration time, t_eq, which means how long the sample is isothermally kept for recovery of the thermal homogeneity after heating, were chosen. The dielectric anomaly was observed when measured with in t_eq of 600 s and 900 s with T_ini = 180 K, suggesting the emergence of a noble metastable state with a high dielectric permittivity.