This paper presents a new method for deciding the supply-air temperature setpoint of VAV (Variable-Air-Volume) system of air-conditioning by vote. Conventional VAV control resets the supply-air temperature setpoint by a set of constant variation-ratios. These systems, however, suffer from significant problems. (1) How to decide the initial value of the supply-air temperature setpoint when VAV system starts running. (2) How to set appropriate variation-ratio of the suply-air setpoint. (3) How to respond demands when upper supply-air temperature and lower supply-air temperature are simultaneously offered. (4) How to link supply-air temperature control with supply-air volume control. The new method solves such problems. The vote method employs control logic that chooses to either minimize the system deviation of room temperature control or minimize supply-air volume according to its control objective. The vote method may also link supply-air temperature control with other control strategies, such as minimizing chilled/hot water volume control logic in the neutral zone of cooling/heating action. This paper also demonstrates effective graphical display of the vote method. The graphical display enables simple expression of such items as the decision's result and its influences, the current status and the importance of each VAV end unit, and selection ranges of supply-air temperature setpoint. These items are not easily expressed by words or numbers. Comparative experiments using a real system in a commercial building prove the controllability, stability and practicability of the vote method, and shows its advantages over conventional methods.