Methods and apparatus are provided for releasing an Internet Protocol (IP) address by a Packet Data Network (PDN) GateWay (PGW) in a mobile communication system. The PGW allocates, to a User Equipment (UE), an Internet Protocol version 4 (IPv4) address and an Internet Protocol version 6 (IPv6) address to activate a PDN connection. If the UE releases the IPv4 address: the PGW identifies whether a PDN type associated with the PDN connection is an IPv4/v6, which indicates that the UE is allocated both the IPv4 address and the IPv6 address; the PGW transmits, to a Serving GateWay (SGW), a Binding Revocation message including information indicating release of the IPv4 address with the PDN connection remaining if the PDN type is the IPv4/v6; and the PGW receives, from the SGW, a Binding Revocation acknowledge message to acknowledge release of the IPv4 address.