A novel symbol timing synchronization algorithm based on Frank-Hermiller (FH) sequence is proposed for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems with carrier frequency offset. The timing synchronization is realized by using only one training symbol which is composed of conjugated symmetric sequences. The timing estimation of the proposed algorithm is robust to poor frequency offset. The proposed algorithm is shown to eliminate the timing sidelobes of Park’s algorithm and has a more accurate timing estimation. In the condition of frequency offset, the timing metric of the proposed method still maintains its peak value at the correct timing point, while the values are almost zero at all other positions.