The main disadvantage of OFDM systems is its sensitivity to carrier frequency offset (CFO) and timing offset. In this paper, an efficient method for estimating CFO is developed based on maximum likelihood principle, of which emphasis on the CFO ambiguity resolution. In this method, only one training symbol is required and the range of the CFO which can be handled is the overall transmission spectrum. Differential coding in frequency-domain is performed to estimate the integral part of CFO (IFO). When frequencyselective channel is present, a prior information of the statistic of the channel frequency response is used to eliminate its effect on estimation performance. Computer simulations are performed to show the superiority to the method available1.