In this paper we propose architecture and design methodology for the Inter-Domains Security Management (IDSM) Model for the IP Multimedia System (IMS) [1] which is standardized by Third Generation Partnership Project (3GPP) [2] and 3GPP2 [3] as Next Generation Mobile Networks. The IDSM Model is based on Trust Domain Relationship between different network domain operators using policy based security associations and managements. The design methodology introduces interdomains security gateways for generating and managing keys and certificates based on Public Key Infrastructure (PKI) architecture. IPSec protocol is used for implementing Confidentiality and Integrity protection. This research work is part of Secure Service Provisioning (SSP) Framework [4] for the IMS Playground [5] and Third Generation Beyond (3Gb) Testbed [6] at the FOKUS Fraunhofer Institute.