Abstract: The impact of Four-Wave- Mixing (FWM) is investigated using the proposed Assign Shortest Path First (ASPF) algorithm for wavelength assignment in Routing and Wavelength Assignment (RWA) in two different topologies. The results show that the ASPF algorithm indulges more FWM crosstalk in a 8 node Ring topology compared with a 8 node Mesh topology for optical channels of 16 and 32. Careful optical channel capacity, low FWM crosstalk power in different network topology are strongly desired for the accomplishment of efficient, cost-effective, high capacity WDM transparent optical network.