Entity migration can help to sustain large scale, long running distributed simul ations. The shortcomings of the high level architecture (HLA), when dealing with this issue, are presented, and the ad hoc approaches for an HLA-compliant feder ate to move entities are analyzed. A protocol used to move entities under HLA/RT I is introduced, then its implementation issues are thoroughly discussed, includ ing the starting of the migration, the entity state transfer mechanism, the mess age relaying and processing logic. The executing framework of the protocol is al so presented. Experiments prove the feasibility of this protocol. Compared with relative studies, this approach can avoid missing messages during migration, and guarantee the correct message process sequence.