An optimal strategy for outdoor air control is developed using a system approach based on prediction to minimize energy consumption. ARMA model is used to predict the energy performance which is expressed by an energy-increment equation. The energy-increment equation is formed to involve the real-time variations of AHU load and energy use of reheaters of VAV terminals. To minimize the Energy-increment equation by genetic algorithm, the optimal settings of outdoor air ratio of AHU and reheating could be obtained. The strategy is tested and evaluated in a simulated environment under various outdoor and indoor conditions. (C) 2005 Elsevier B.V. All rights reserved.