In this manuscript, coupled and Schrödinger equations have been used for theoretically calculating and comparing XPM-induced crosstalk due to 3OD in SCM-WDM transmission system. XPM-induced crosstalk increases from (-95dB to-73dB) using Coupled equations and from (-117dB to-86dB) using Schrödinger equation when modulation frequency is increased from 0.5 to 5GHz.It is observed that XPM induced crosstalk is increased from (-90 dB to-67 dB) using Couple equation and from (-120 dB to-79 dB) using Schrödinger equation when transmission length lies between 5 to 50 km. XPM induced crosstalk remains between (-70 dB to-45 dB) using Couple equation and between (-110 dB to-60 dB) using Schrödinger equation when power lies between 0.2 to 2 mW. It has been deduced that Schrödinger equation gives less non-linear crosstalk induced by XPM in comparison with coupled equations in SCM-WDM transmission system.