Ionic liquids 1-butyl-3-methylimidazolium acetate [C₄mim][OAc] and 1-hexyl-3-methylimidazolium acetate [C₆mim][OAc] were prepared by the neutralization method and confirmed by ¹H NMR spectroscopy and differential scanning calorimetry (DSC). Using isothermogravimetrical analysis, the enthalpy of vaporization, Δ^g_lH^°_m(T_av), at the average temperature, T_av, for [C_nmim][OAc] (n = 4, 6) were determined. Using the established method, the difference of heat capacities between the vapor phase and the liquid phase, Δ^g_lC^°_pm, for [C_nmim][OAc] (n = 4, 6) were calculated based on the statistical thermodynamics. Therefore, the value of Δ^g_lH^°_m(T_av) can be transformed into Δ^g_lH^°_m(298) by using Δ^g_lC^°_pm. The vaporization enthalpies were also further tested by molecular dynamic (MD) simulations, and the results were in good agreement with the corresponding experimental values. In terms of the new scale of polarity for ILs, the order of the polarity of [C_nmim][OAc] (n = 4, 6) was also predicted and good agreement with our experiences.