Since the timing and frequency synchro nizations are critical steps in the signal processing of the coherent optical or thogonal frequency-division-multiplexing (CO-OFDM) system,modified methos are proposed for timing and frequency synchronizations respectively in the CO-OFDM system in this paper.The modified timing synchronization method uses a modified training symbol weighted by a pseudo-noise (PN) sequence and solves the ambigui ty problem in Park′s method,while the modified frequency synchronization method uses a single training symbol to implement both the partial and the integer fre quency offset estimations.and maintains equivalent accuracy compared with schmid l′s method which needs two training symbols.The modified synchromization scheme is tested in a 20Gbit/s 256-subcarrier quadrature phase shift keying (QPSK) modulated CO-OFDM system by Matlab simulation,and the results show that the mod ified scheme gives a more accurate timing estimation and a more economical frequ ency offset estimation in a wider frequency range.In addition,with the aid of th e chromatic dispersion (CD) compensation based on the results of the CD monitori ng before the synchronization steps,the modified timing synchronization works mo re stably,and the subcarrier recovery achieves better performance,reducing the o ptical signal-to-noise rate (OSNR) requirement by 3dB at the bit error rate (BER) of 10－3．