In this paper, we propose a timing synchronization method for orthogonal frequency division multiplexing (OFDM) systems by exploiting the averaged differential correlations of preamble symbol. For the designed simple preamble whose second half-symbol is the complex conjugate of the first one in the time-domain, the proposed timing synchronization method has an impulse-shaped timing metric. Simulation results indicate that the proposed method improves the timing estimate mean square errors (MSE) performance remarkably than conventional methods at low signal-to-noise ratio (SNR) in multipath fading channel. To reduce the computation complexity of the proposed method, we suggest modifying the timing estimation by simplifying the amplitude information of OFDM signals in polar coordinates.