Friction stir spot welding (FSSW) was applied to lap joining of aluminum alloy sheets and steel sheets. A 1.2 mm-thick non-plated carbon steel sheet and plated steel sheets with zinc alloy (ZAM), pure zinc (GI), zinc alloy including Fe (GA) and Al-Si alloy (AS) were prepared. The melting temperature of the plated layer is 330, 420, 880 and 640°C, respectively. A 1.1 mm-thick 6022 aluminum alloy sheet was overlapped on the steel sheet. A rotating tool was inserted from the aluminum alloy sheet side and the probe tip was kept at the position of 0.2 mm above the lapped interface for 3 seconds. For ZAM and GI, original plated layers were removed from the interface and intermediate layers were formed at the joint interface. This is because the melting temperature of the plated layer was lower than the interface temperature under the rotating probe tip during the FSSW. In contrast to that, the partial original plated layer remained after welding, and additional layer formed at the plated layer /aluminum alloy interface for GA. For AS, Al-Fe intermetallic compound layer, which was formed at the original Al-Si alloy plated steel surface remained.