Four-wave mixing (FWM) tone degrades system performance in dense wavelength-division-multiplexed (DWDM) optical transmission. A method of evaluating the efficiency of non-degenerate FWM tone for return-to-zero modulation scheme is developed without ignoring group-velocity walk-off of random bit sequence between channels. A theoretical model evaluating the standard deviation of FWM tone is then given. Some calculation examples show that, when the duty cycle and channel separation become small, the initial relative time delay of random bit sequences in different channels and fiber dispersion are more important to the FWM noise. Choosing optimal values of them is important to make the system to have low dispersion and small FWM crosstalk.