A method for determining a number of ranging slots by a base station (BS) in a broadband wireless access (BWA) communication system. The method includes counting the number of mobile stations (MSs) that succeeded in ranging in a current frame, determining a ranging slot occupation ratio in the current frame, comparing a ranging slot occupation ratio of a previous frame with the ranging slot occupation ratio of the current frame, detecting an increase in the number of ranging-requesting MSs, if the ranging slot occupation ratio of the current frame is greater than the ranging slot occupation ratio of the previous frame, and determining the number of ranging slots of a next frame such that the number of ranging slots of the next frame is greater than the number of ranging slots of the current frame.