A thermally coupled distillation system is constructed using two columns used in a conventional two column system, and its operability is examined by investigating the dynamic behavior. For the control of three product specifications, the step response test is performed and a 3×3 control structure using flow rates of overhead and side products and vapor boilup is suggested. The performance of the proposed control system indicates that the specification control of bottom product is the most difficult but the operation of the proposed system is available.