Live migration of virtual machines (VM) has recently become a key ingredient behind the management activities of cloud computing system to achieve the goals of load balancing, energy saving, failure recovery, and system maintenance. However, to our knowledge, most of the previous live VM migration techniques concentrated on the migration of a single VM which means these techniques are insufficient when the whole virtual cluster or multiple virtual clusters need to be migrated. This paper investigates various live migration strategies for virtual clusters (VC). We first describe a framework VC-Migration to control the migration of virtual clusters. Then we perform a series of experiments to study the performance and overheads of different migration strategies for virtual clusters, including concurrent migration, mutual migration, homogeneous VC migration, and heterogeneous VC migration. After that, we present several optimization principles to improve the migration performance of virtual clusters. The HPCC benchmark is selected to represent the virtual cluster workloads, and the metrics such as downtime, total migration time, and workload performance are measured. Experimental results reveal some new discoveries which are useful to the future development of new migration mechanisms and algorithms to optimize the migration of virtual clusters.