This invention provides a method and apparatus that uses a mobility management server (MMS) device for supporting mobility management in wireless networks. With the MMS device for services of resource management and packet relay, this invention speeds up the handover procedure for a mobile device switching from a first network domain to a second network domain in a wireless network environment having a session initiation protocol (SIP) server. When the mobile device needs to switch to the second network domain, the MMS allocates the required resources for packet relay, provides the related information for the second network domain, and takes care of the packet relay. This invention shortens the inter-domain handover latency and reduces the number of lost packets during the handover procedure. Thereby, the transmission efficiency of the present invention meets the requirement for real-time multimedia applications.