In order to overcome the defects of least squares (LS) estimator used to defend chromatic dispersion in the direct-detection (DD) optical orthogonal frequency division multiplexing (OOFDM) transmission system, an adaptive least mean square (LMS) estimator is presented owing to its brief calculation and easy signal block processing. The performance of the proposed estimator is evaluated in an experimental system with 2.5 Gb/s DD-OOFDM signal at back-to-back (BTB) and transmitting over 100 km standard single mode fiber (SSMF). The experimental results show that the power penalty of system using LMS equalizer is 2 dB less at 10×10-2.5 bit error rate (BER) and 2.5 dB less at 10×10-2.0 BER than using LS equalizer when the OFDM signals transmitted by BTB and 100 km SSMF. It is proved that the system using LMS equalizer has a better performance to restore signals than using LS equalizer.