There is an increasing interest on OFDM (Orthogonal Frequency Division Multiplexing) recently, because of its efficient use of the frequency spectrum and well multi-path tolerance, which are desired qualities especially for wireless channel. However, its disadvantage is sensitivity to frequency error. Thus to detect sub-carrier frequency correctly and conveniently is the key problem for OFDM system. This paper presents an asymmetric base-band modulation scheme for OFDM system, which makes it easier to estimate sub-carrier frequencies than conventional symmetric base-band modulation. Although this approach has its drawback of increasing BER, it can be compensated successfully by hexagonal arrangement of constellation points. Simulation results have proved that with the compensation, asymmetric base-band modulation doesn't weaken the system resistibility to noise.