Friction Stir Welding (FSW) is a modern solid-state welding technique invented in 1991 at the Welding Institute of Cambridge. It is becoming popular and efficiently used in many applications including automotive, marine, railway and aircraft industries. FSW is vulnerable to the existence of various kinds of defects that may arise from the inappropriate selection of weld parameters. Buried defects such as voids and tunnels necessitate the existence of appropriate non-destructive evaluation (NDE) techniques. Guided waves are ultrasonic elastic waves that have the ability to travel over long distances. Their ability to propagate in complex structures and in both metallic and nonmetallic materials, has drawn the attention of many researchers to explore the potential of their use in detecting surface and internal flaws. However, defect-inspection in dissimilar friction stir welded joints is associated with several challenges related to the difference in the attenuation behavior between the two mediums. The purpose of this work is to scrutinize the interaction behavior of Lamb waves with friction stir welded joints of dissimilar materials. An AZ31B/AA6061-T6 FSW plate was used for investigation. The frequency-wavenumber filtering technique of the data extracted from a laser Doppler vibrometer (LDV) test, was used to filter out the reflected wave from the incident and transmitted waves. Reflection was observed from the weld zone of the used plate. A sensor network was designed to capture the drop of the incident signal, straight before and straight after the weld zone. This drop was used to measure the reflection generated upon the interaction of guided waves with the weld. The results showed that the average reflection, of the incident wave, generated when the wave crosses from AZ31B to AA6061-T6 base metal was around 10% higher in comparison to when the wave crosses from AA6061-T6 to AZ31B.