An apparatus and method for estimating a frequency offset in an Orthogonal Frequency Division Multiplexing (OFDM) system are provided. Two time domain signals are generated from one received OFDM symbol through two observation windows with different delay times. The two time domain signals are compensated for a candidate frequency offset. A discrete Fourier transform process and a phase compensation process in a frequency domain are performed. A difference between two frequency domain signals compensated for a phase difference is computed with respect to each candidate frequency offset. For all candidate frequency offsets, difference values are compared with each other. A candidate frequency offset mapped to a minimum difference value is detected as an optimal frequency offset. The frequency offset can be more exactly estimated without consuming a bandwidth and increasing hardware complexity of a receiver.