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Control of internal combustion engines depends on reduced idle speed and stabilized idling to improve fuel consumption.. Using a valve (ISC/V) to adjust air intake at idling, various idle speed control techniques have been proposed to improve response to variations in the target idle speed due to disturbance and to enhance speed control stability. Many conventional idle speed control systems utilize P and I control. They cannot compensate for the phase difference caused by the intake air volume between the throttle and intake valves. This leads to poor response to variations in the target idle speed and unsatisfactory control accuracy, that is, problems in preventing engine stalling and maintaining idle speed stability. To solve these problems, an idle speed control system based on fuzzy control(1)* theory is proposed. However, it is impossible to determine the control constant (membership function) theoretically.