Virtual　machine(VM)　live　migration　is　the　key　technology　for　next　generation　of　IaaS　cloud　services.　There　exists　some　tradeoff　between　minimizing　the　copy　phase　duration　and　maintaining　an　acceptable　quality　of　service.　In　this　paper,　a　network　bandwidth　cost　function　model　is　built　between　migration　and　service　based　on　queuing　theory.　From　the　experimental　results,　the　model　has　a　perfect　fitting　with　the　actual　measurements.　©　2012　IEEE.