Abstract: Migrating entities are beneficial to improve performance of large-scale distribu ted simulation systems, while time synchronization during migration is the key a nd difficult issue. The time synchronization mechanism for migrating entities in HLA (high level architecture) federated simulation is studied. Transition of fe derates' states is analyzed, and synchronization strategies for the old federate to relay messages and for the new federate to receive and process messages are proposed. Under these strategies, messages are processed in time-stamp order, th e logical time of both sides is synchronized, and messages are never lost or dup licated. Message retraction is also supported to enable optimistic synchronizati on.