Recently, the interest on welding of magnesium alloys has been increasing rapidly in automotive and aerospace industries owing to their excellent properties such as light weight, high specific strength and stiffiness. Fusion welding of these alloys is not preferable due to hot cracking, formation of porosity etc. However solid state welding techniques, such as, friction stir welding (FSW) are found to offer solution to the above problems. In this study, an attempt was made to understand the effect of tool rotational speed of FSW process on microstructure and tensile properties of AZ31B magnesium alloy. FSW joints were fabricated using different levels of tool rotational speed between 1200 and 1800. Tensile properties of the welded joints were evaluated and correlated with the weld zone microstructure and hardness. From this investigation, it is found that the joints fabricated using a tool rotational speed of 1600 rpm with a welding speed of 0.5 mm/s and an axial force of 4 kN yielded superior tensile properties compared to other joints. Formation of finer grains and higher hardness in stir zone are the main reasons for the superior tensile properties of these joints. © 2017 Elsevier B.V., All rights reserved.