An apparatus and method support various system bandwidths in a broadband wireless communication system. A Mobile Station (MS) operation includes determining a system bandwidth through a primary preamble for frame synchronization, which a Base Station (BS) transmits. A Fast Fourier Transform (FFT) size, the number of guard subcarriers, and the number of used subcarriers corresponding to the system bandwidth are determined using a first table. An operation parameter of a modem is set according to the FFT size, the number of guard subcarriers, and the number of used subcarriers.