The present invention relates to a method for generating/changing a transport connection identifier (CID) in a portable Internet System, and a terminal using the same. Accordingly, the present invention provides a terminal including a packet detecting unit, a user input/output unit, a CID managing unit, and a data storing unit. The packet detecting unit determines whether a packet can be classified by a pre-established packet classification rule or a packet cannot be classified by the rules (5304). For the packet that cannot be classified by the pre-established packet classification rule, the user input/output unit receives QoS information from a user (5322, 5324). The CID managing unit generates a new packet classification rule for the unclassified packet (5326), checks a CDD in a data storing unit (5328), the CID having the same QoS class as that of the QoS parameter input through the user input/output unit, and determines whether to generate a new CID (5340) or to add the new packet classification rule to the pre-established CID (5330) depending on the checking result. The data storing unit storing unit Stores the new packet classification rule generated by the CID managing unit and information on the CID (5354).