A method is provided for allocating sub-channels in an Orthogonal Frequency Division Multiple Access (OFDMA) mobile communication system including a plurality of cells or a plurality of sectors. The method comprises dividing a time interval preset into a band Adaptive Modulation and Coding (AMC) sub-channel region and a diversity sub-channel region, for each of the cells or sectors; and allocating band AMC sub-channels only in a preset frequency region of the full frequency band used in the OFDMA mobile communication system, in the band AMC sub-channel region.