The first order design constraint in dense packaged clusters is power consumption. The currently developed cluster systems are conservatively designed so that the expected peak power does not exceed the power limit. However, practical power consumption seldom reaches the peak power. In this paper, we propose a new approach to design a high performance cluster system by an adaptive power control technique. Our approach is to integrate many computation nodes into a system whose total theoretical peak power exceeds the limit and to control runtime effective power by optimizing the number of working nodes and/or the clock frequency of the processors. We show the algorithm of the adaptive power control and performance evaluation by using a real cluster system. Evaluation results show that our proposed approach greatly improves performance as large as 46% compared to a conventional cluster system.