Since the traditional timing synchronization algorithms based on compl ex pseudo noise (PN) sequence have ambiguity timing estimation and a high peak to average power ratio (PAPR) ,we propose a timing synchronization algorithm based on even symmetry constant amplitude zero auto－correlation (ES-CAZAC) sequences for coh erent optical othogonal frequency division multiplexing (CO-OFDM) system.This a lgorithm relies on the good properties of CAZAC sequence:low PAPR and even symmetry of even length sequence.The training sequence consists of two different CAZAC sequences,which are regarded as two windows respectively.After using the even symmetry property to calculate the correlation degree of the two windows,we can get the correct timing point only when the two windows have a maximized correlation degree simultaneously.The modified algorithm is tested both in a Gaussian white noise channel and 10a Gbit/s CO-OFDM system.The results show that the modified algorithm gives a sharp timing metric function without increasing a ny complexity.In the CO-OFDM system,compared with Park′s,the degree of side lobe ambiguity for modified algorithm decreases by 26.7%.In a large dispersion system,the mean square error (MSE) of the modified algorithm maintains 10－5,w hich mean s a correct timing estimation.At the same time,the ES-CAZAC sequence has an ideal PAPR value of 0dB.The modified algorithm with a l ow complexity,an ideal PAPR and a high accuracy is suit able for the high speed CO-OFDM system.