Lap joining of A6111 alloy and steel (SPCC: Steel Plate Cold-rolled C) plates was performed using a defocused YAG laser beam. A detailed investigation was performed on the intermetallic compound (IMC) layer formed at the weld interface. Two representative joints fabricated under different welding conditions were selected and the effect of the welding conditions on the kind and morphology of the IMC was investigated using a transmission electron microscope (TEM). An electron diffraction pattern method was used to identify IMC. It was found that the morphology and kind of IMC formed at the weld interface were strongly affected by the welding conditions, in particular, by the amount of heat input during welding. The thickness of the IMC layer formed at the weld interface was about 1 μm and the average grain size of the IMC in the layer was less than 300 nm when the joining was carried out with a small amount of heat input. The IMC layer was composed of Fe3Al, FeAl, Al2Fe, Al5Fe2 and Al13Fe4 in this case. However, the thickness of the IMC layer was around 6 μm when the joining was carried out under high heat input conditions. In this case, the IMC layer was composed of coarse Al5Fe2 (5 μm) and Al13Fe4 (1 μm). Therefore, it is considered that the reduced bonding strength of the joint with a thick IMC layer is due not only to the overall morphology of the IMC layer but also to the formation of coarse Al-rich IMCs in the layer.