This paper presents the results of an experimental study aimed at assessing the tensile strength and fatigue behaviour of friction stir welded (FSW) AZ31B magnesium alloy. The effect of rotational and linear welding velocities on the weld microstructure and performance are discussed. The FSW joints were made in a conventional milling machine adapted for the process. The fatigue tests of the welded joints were carried out under pulsating tension conditions to develop a S-N curve. The salient result from the tests was that the specimens welded with higher rotational to linear velocity ratio, (V R/V A), exhibited the best fatigue behaviour due to the absence of root defects. However, the fatigue strength was reduced by 30% with respect to the base metal.