Coherent optical orthogonal frequency division multiplexing (CO-OFDM) can immun e to chromatic dispersion (CD) and polarization mode dispersion (PMD) in fi ber links.However, the large peak-to-average power ratio (PAPR) of OFDM signals makes it suscepti ble to fiber nonlinearities and consequently degrades the transmission performance.In this p aper,an approach based on constant envelope (CE) modulation,which can lower the PAPR of the opti cal signal to 0dB,is proposed to improve the fiber nonlinear performance for CO-OFDM systems.Simula tion results show that after 800km standard single-mode fiber (SSMF) transmission,the maximum launch power of the proposed 40Gbit/s CE-CO-OFDM system with 16-QAM subcarrier modulation is imp roved by 6.2dB, 9.3dB and 10.9dB compared with conventional CO-OFDM for the dispersion map w ithout dispersion compensation fiber (DCF),the 100ps/nm under-compensated and the fully periodi c compensation dispersion maps,respectively,although an optical signal-to-noise ratio (OSNR) penalty of ~1.8dB is observed in the linear region of transmissions.When the scheme is utilized in the hybrid wavelength division mult iplexing (WDM) transmission of the 40Gbit/s CO-OFDM channel with 10Gbit/s NRZ-OOK channels,a 5.2dB increase of the maximum launch power is also observed.Therefore,the proposed scheme can effectively improve the fiber nonli near tolerance for CO-OFDM systems in different transmission scenarios.