The Petlyuk process is known as a prospective energy-saving distillation process, which consists of a prefractionator and a main column with a side stream, connected by two vapor/liquid streams. Some of the modified structures by adding a reboiler or a condenser to the prefractionator have been proved to provide good operability and controllability. In this paper, we propose necessary conditions for flow rates of the interconnecting vapor and liquid streams which give better separation performance for a modified Petlyuk process equipped with a reboiler at the bottom of the prefractionator. The separation characteristics compared with a conventional two-column process and the original Petlyuk process are also presented by numerical examples. The simulation results demonstrate that the proposed necessary conditions give useful information to reduce the amount of optimization calculation and that the energy saving performance of the modified Petlyuk process is close to that of the Petlyuk process when the feed composition of the middle component becomes larger.