Al-Mg alloy A5052 and low carbon steel SPC270C joints by the friction stir spot welding (FSSW) were used for fatigue tests. From the experimental results, it was found that fatigue strengths and transition in fatigue fracture mode of the joints depended on thickness of A5052 beneath FSSW region rather than plunge depth condition. The net shear strength per unit joining area was almost equal irrespective of the joining condition. Finally, it was clarified that fatigue property of the dissimilar FSSW joint used in this study was significantly influenced by equilibrium in three kinds of independent strength which were a shear strength at joining interface, a fatigue strength of partial A5052 beneath the welded area and a fatigue strength originated at joining gap between A5052 and SPC270C around periphery of the welded region.