The far end crosstalk (FEXT) saturation of microstrip differential signal pair is investigated in this paper. It is found that coupled length, rise time of attack signal, spacing between differential signal pair and attack line, coupling level of differential line can significantly affect FEXT of differential noise which causes serious signal integrity issues. In the situation when the FEXT of differential noise reaches saturation, the minimum coupled length is mostly affected by the rise time of attack signal. Also, coupling level of differential line and spacing between differential signal pair and attackline are two major factors changing saturate value of FEXT.