MEK(methyl ethyl ketone) cannot be separated from water by normal distillation because of the presence of minimum boiling azeotrope. However it can be separated by azeotropic distillation by adding entrainer. The best entrainer was selected by considering the ratio between infinite dilute activity coefficients of each component for suggested entrainers and relative volatilities. From this method, acetone was selected as the best entrainer for the MEK-Water system. When the distillation tower was simulated by the UNIFAC equilibrium model for this MEK-Water-Acetone system, the minimum reflux ratio was observed as 0.6. Also, the optimum feed-plate was calculated at each reflux ratio for various amounts of entrainer. Results indicated the number of ideal plates decreases with the amount of entrainer. As the best results, 13 ideal plates were required to obtain 95 mole% of MEK as the bottom product from 70 mole% of MEK of the MEK-Water solution when the reflux ratio and the ratio of entrainer to feed were 0.85 and 2, respectively. In this case, the optimum feed-plate location, which required the minimum number of ideal plates, was the eighth.