Abstract: Cloud computing is a novel paradigm for the provision of service on demand. Through the use of virtualization, multiple virtual machines(VMs) are allowed to run on the same physical server. Aiming at the problem that multiple VMs shares the same underlying physical resources, this paper presents a model for performance analysis of virtualization-based cloud centers. The model considers the resources sharing among VMs. In addition, a service request is divided into many subtasks, which is much closer to the reality of a cloud system. The performance indicators such as blocking probability, the average response time are obtained.