Dynamically allocating a frame cell (FC)/subchannel in a mobile communication system by an access point that receives channel quality information (CQIs) fed back from access terminals, determines a modulation and coding scheme (MCS) to be applied to the access terminals based on the CQIs, and if access terminals whose FCs/subchannels must be changed are detected from the access terminals, sends an FC/subchannel change request to an access router. The access router allocates an FC/subchannel set by selecting a number of FCs/subchannels according to the FC/subchannel change request, and transmits information on the allocated FC/subchannel set to the access point. The access point selects and allocates a particular FC/subchannel from among FCs/subchannels in the FC/subchannel set information to the access terminals whose FCs/subchannels must be changed, based on CQIs last received from the access terminals.