The mixture of acetonitrile (1) + n-propanol (2) forms a binary azeotropic mixture at atmospheric pressure, which cannot be separated by common distillation. In this paper, the ionic liquids (ILs) 1,3-dimethylimidazolium dimethylphosphate ([MMIM][DMP]) and 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM] [DEP]) were used as solvents for separating the binary azeotropic mixture. The ternary isobaric vapor liquid equilibrium data of acetonitrile (1) + n-propanol (2) + ILs (3) at atmospheric pressure in three IL mole fractions of 0.048, 0.091, and 0.131 were obtained. The results showed that the addition of [MMIM][DMP] or [EMIM][DEP] could enhance the relative volatility of acetonitrile to n-propanol, and the azeotropic point of the binary azeotropic system could disappear at the mole fractions of ILs above-mentioned. The effect of separation of ILs was in the order of [EMIM] [DEP] > [MMIM][DMP]. The interaction parameters were obtained by fitting the nonrandom two-liquid (NRTL) model to the binary vapor-liquid equilibrium experimental data. The predicted results obtained using the NRTL model were in good agreement with the experimental data. In addition, a quantum chemical calculation based on Gaussian 09 at the B3LYP/6-311++G** level was also conducted. The calculated results indicated that the molecules of ILs had a stronger interaction with the n-propanol molecule than with the acetonitrile one.