Fiber four wave mixing (FWM) is studied for multi channel,multi amplifier WDM systems composed of fibers with different length and dispersion.By using a derived general theoretical model,the FWM of several systems is analyzed.The results show that the model is important in WDM systems design.It is also proved that the combination of single mode fiber,which is used as transmission fiber,and high negative dispersion fiber,which is used as dispersion compensation,is an effective way to decrease the fiber FWM.