1. An apparatus for compensating for an I (in-phase)/Q (quadrature-phase) mismatch in a wireless communication system, the apparatus comprising: a detector for detecting transmission signals and providing the transmission signals to a reception path; a Fast Fourier transform (FFT) operator for performing FFT on a detected transmission signal; and an I/Q mismatch estimator for estimating the I/Q mismatch using a preamble of the transmission signal after the FFT.