A novel symbol timing synchronization scheme for direct-detection optical orthogonal frequency division multiplexing (DD-O-OFDM) systems is proposed.Mak ing use of the non-periodic auto-correlation complementary property of the Golay pair, the new algorithm processes the signals in the double sliding window to capture signal e nergy.It processes the signal energy by designing symbol timing function so that the timi ng function can reach the maximum to get the correct symbolic timing point while capturing signal energy.The performance of the new algorithm is evaluated in terms of mean and m ean-square error (MSE) in one experimental system with 1.5G bit/s D D-O-OFDM signal transmission over 200km standard single mode fiber (SSMF).The experimental results show that th e new algorithm has smaller MSE than other algorithms and achieves higher timing estimation accuracy.When the received power is less than －36dBm,the new algorithm can st ill obtain the smallest timing estimation mean and MSE,while the perfor mance of other algorithms significantly decreases,so the timing precision of the new algorithm is the hig hest. Meanwhile,in the case of low power of －40dBm,the new algorithm can still accur ately ca pture the timing point,and reduce the sensitivity on dispersion of th e system timing synchronization.