It is critical to reduce the electric energy consumed by servers in clusters. In cloud computing systems, computation resources like CPU, memory, and storages are virtualized so that applications can use the resources without being conscious of which physical servers supports which service. Virtual machine technologies are now used to support applications with virtual computation service. Virtual machines are used to perform application processes issued by clients. More importantly, virtual machines can migrate from a host server to a guest server while processes are being performed on the virtual machines. In this paper, we take advantage of migration technologies of virtual machines to reduce the electric energy consumed by servers in a cluster. By using the virtual machines, we propose an energyaware migration model of processes to decide to which virtual machine a process issued by a client is allocated and which virtual machine migrates to another energy-efficient server in order to reduce the electric energy consumption.