The high peak-to-average power ratio is the main disadvantage of Orthogonal Frequency-division Multiplexing (OFDM) communication systems, so there is the same problem in optical OFDM systems. A joint reduction Peak-to-average Power Ratio (PAPR) scheme, based on combining Hadamard transform with clipping, is successfully applied to an experimental system of direct-detection optical OFDM signal transmission over fiber. The performance of bit error rate of system is mainly researched. In this experiment, the 2.5 Gb/s QPSK optical OFDM signal with Hadamard transformation is generated and transmitted over single mode fiber. The experiment results show that the proposed scheme can reduce PAPR almost 3.2 dB. Meantime, the received sensitivity is improved 3dB with 100 km fiber transmission compared to that of ordinary optical direct detection OFDM system.