The ionic liquids(ILs)[Cnmim][Pro](n = 4,5,6)(1-alkyl-3-methylimidazolium propionate)were prepared by the neutralization method and characterized by 1H-NMR spectroscopy and differential scanning calorimetry(DSC).Using isothermogravimetrical analysis,the vapor pressures in the temperature range of 423.15~463.15 K and the enthalpy of vaporization,△g lHo m(Tav),at the average temperature,Tav,for these ILs were determined [1-2].The difference of heat capacities between the vapor phase and the liquid phase,△g lCp o m,for [Cnmim][Pro](n = 4,5,6)were calculated based on the statistical thermodynamics using Verevkins method.Therefore,in terms of △g lCp o m,the value of △g lHo m(Tav)can be transformed into △g lHo m(298).Based on Clausius-Clapeyron equation,the hypothetical temperature of normal boiling point(NBP),Tb for [Cnmim][Pro](n = 4,5,6)were estimated.In terms of Tb,the values of entropy and the Gibbs free energy of vaporization for these ILs were predicted at different temperatures.Based on Hildebrands theory a new method to predict the polarity of the ILs was put forward.The order of the predicted polarity for the ILs is in good agreement with the experiment value.