CO-OFDM has the advantages of both 'coherent detection' and 'OFDM modulation' and provides many merits for future high-speed fiber transmission systems. In this paper, we analyze the performance of Coherent Optical OFDM (CO-OFDM) for different data rates like 1Gbps, 20Gbps and 100Gbps. 4 QAM-OFDM is generated, modulated using MZM and transmitted through the fiber for a distance of 250km. Due to the different nonlinearities of fiber the signal is distorted. At the receiver end the filtered optical signal is converted to electrical signal and OFDM is demodulated. Results shows that at the receiver as the data rate increases, the distortions in the OFDM spectrum and QAM constellations also increase.