In this paper, for the first time, the transmission performances of long-haul 100-Gb/s direct detectionoptical OFDM (DDO-OFDM) and coherent optical OFDM (CO-OFDM) wavelength division multiplexing(WDM) systems are compared by simulation. It provides specific guides for system parameter selectionto get a high-performance and cost-effective OFDM WDM system. Specifically, the comparison involvesthree aspects: launched power is investigated to achieve better system performance; laser linewidth isnumerically investigated to choose cost-effective laser; system dispersion tolerances with different laserlinewidths are analyzed to further reveal the advantages and disadvantages of these two detecting methods,direct detection and coherent detection, in long-haul OFDM WDM system.