Provided is a channel mode converting method of a wireless portable Internet system. When a diversity channel mode is converted into an Adaptive Modulation and Coding (AMC) channel mode, a radio resource for transmitting Channel Quality Information (CQI) is allocated without responding to a resource allocation request and a request between a terminal and an access point. Therefore, a delay time can be reduced on the channel mode conversion. In addition, a CQI channel for transmitting the existing channel quality information is released and a CQI channel for a new channel is allocated on the channel mode conversion. As a result, a time for changing the diversity channel CQI transmission to the AMC channel CQI transmission at the physical layer can be easily synchronized for transmitting the REP_RSP at the MAC.