A system is provided for transmitting/receiving downlink resource allocation information in a communication system. In the system, a base station generates first information including downlink resource allocation information necessary for supporting a first mode supported by the base station, generates second information including length information of the first information, and transmits the first information and the second information. A mobile station notifies the base station of a second mode supported by the mobile station, receives, after the notification, the first information and the second information from the base station, and skips without decoding as much downlink resource allocation information as a length corresponding to the length information without decoding, if the first mode is different from the second mode.