Power management of web server clusters have become a critical design issue because of its increasing power consumption and cooling cost. Current web server clusters are normally designed to have a performance capacity to handle peak loads, where all servers are fully utilized (turned on and running at maximum frequency). But in practice, peak load conditions rarely or never happen and most of the servers are underutilized for a long time. To make a better control of the web server clusters, we propose a technique which reconfigures the cluster-based web servers by adjusting the number of active nodes and clock frequency of each node according to the load conditions to minimize the power consumption. In this paper, we focus on heterogeneous server clusters since most real-world web-server clusters consist of several homogeneous clusters and construct a heterogeneous cluster. We describe our power-performance modeling and optimization for heterogeneous cluster-based web servers for low power.