The structure of OFDM symbol has great influence on the performance of coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. We present, for the first time, a typical structure of OFDM symbol with zero padding and pilots for CO-OFDM. Virtual subcarriers is used for guard band and eliminating the impact of DC and low frequency subcarriers. Pilots are intended to track the channel variation and compensate for phase noise. Simulation results indicate that CO-OFDM system with this structure is more resistant to the influence of optical carrier, MZ modulator, linewidth, frequency offset than the system without zero padding. It is also found that 32 pilots are enough in one OFDM symbol. Furthermore, the proposed structure is very suitable for frequency offset compensation. Using the structure, performance of CO-OFDM is improved largely.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.