A method for managing mobility of an Access Terminal (AT) using a Proxy Mobile Internet Protocol in a mobile communication system. Access authentication is performed between the AT and an Authentication, Authorization and Accounting server. The AT sends a Dynamic Host Configuration Protocol (DHCP) Server Discover message including information for authentication check to an Access Gateway (AG) while performing the access authentication. The AG sends a Proxy Registration Request message including information for mobility management for the AT to a Home Agent (HA). The AG receives from the HA a Proxy Registration Response message indicating a successful reception of the Proxy Registration Request message. The AG sends a DHCP Offer message including a Home Address (HoA) of a corresponding AT and sends a DHCP Acknowledgement message for accepting use of the HoA, to the AT, when the AT notifies the AG of its intention to use the HoA.