The OFDM system has been a widely adopted technique for both wired and wireless transmission. It is well-known that one of the most important problem for the OFDM system is the carrier frequency mismatch between the transmitter and the receiver. The frequency mismatch is always modeled as a CFO(carrier frequency offset) effect. Therefore how to estimate the CFO correctly is one of the most important problems for the OFDM systems. In this thesis, we will use a semiblind method for estimating the CFO in the OFDM systems. At the transmitter, some tones of the transmitted signal before applied to the normalized IDFT block are chosen to be the virtual carriers. The method is semiblind because we only need to know the locations of these virtual carriers but no need to know what modulation scheme that the transmitted data uses. Using these virtual carriers, we can get our cost function. By minimizing the cost function, the CFO can be estimated. We convert the problem of minimizing the cost function into the problem of finding the GCD(greatest common divisor) polynomial. The method we use for finding the GCD polynomial is the subspace method. Finally the performance of the proposed method will be compared with the method that minimizes the cost function directly.