Abstract: We have theoretically and experimentally investigated the timing offset estimation methods based on training symbol with three different structures (Two-segments, Four-segments and Eight-segments) in direct-detection (DD) optical orthogonal frequency division multiplexing (OOFDM) systems. The performance of the proposed method is evaluated in terms of mean and mean-square error (MSE) of timing offset estimation in one experimental system with 4 Gbits sec^-1 DD-OOFDM signal transmission over 100 km standard Single Mode Fiber (SMF). The experimental results show that the two-segments timing offset estimation method has smaller MSE than the other methods and achieves higher acquisition probability of timing synchronization in DD-OOFDM transmission system.