The effect of the vapor split ratio on the operability of divided wall column (DWC) is numerically investigated. Four DWCs for separating a three-component mixture with, respectively, four different compositions are designed and then simulated to show the dependence of a total annualized cost (TAC) of the columns on the vapor split ratio, r(v), which is subjected to change in operation for reason for either the design uncertainty or the possible change in the feed composition. It is shown that the TAC is sensitive to the r(v), and the extent of the sensibility depends on the feed composition in the design. It is also demonstrated that the flexibilities of the four DWCs designed respectively for the different cases over the feed composition change exhibit differently, and a DWC designed for the feed with relatively higher concentration of the light component tends to have higher flexibility over the feed composition change. By the simulation the vapor split ratio is shown to be an important decision variable in the design of a DWC with uncertain feed composition.