This paper proposes a novel session setup mechanism in IMS (IP Multimedia Subsystem) by introducing the functional entity termed VSS (Visitor Subscriber Server) which is used to provide registration and location management.The new mechanism aims at improving session setup procedure by a betterment of location management in IMS.Through the use of two-tier subscriber information database constituted by the HSS (Home Subscriber Server) and the VSS,the new mechanism seperates the registration procedure and location management from the S-CSCF (Serving-Call'Session Control Function),and makes them be handled by the VSS.At the same time,the new mechanism updates the signaling procedure of registration and session setup.With VSS based location management.the cost of registeration and session setup procedure as well as the delay of session setup procedure become lower.he cost and delay functious of the new session setup mechanism as well aus the cost function of new registeration are deduced.Then,simulations and analysis verify that the performance of this new mechanism gets the benefit in cost and time.