The metallographic factor controlling the strength of friction-welded interface of Al-Mg alloy 5052 to mild steel S10C has been investigated in nano-scale mainly by TEM observations. The bond strength, estimated from the tensile strength of a specimen with a circumferential notch at the interface, increased rapidly with an increase in friction time and then reduced when the friction pressure was 20MPa. A maximum strength of 327 MPa was obtained at a friction time of 4 s (rotation speed = 20 s-1 and forge pressure = 230 MPa). At a friction time of 1 s, the interface consisted of areas where an IMC(Intermetallic compound) layer about 30 nm wide was formed and areas where no IMC layer could be detected. In addition, an Al oxide film of a width less than 10 nm was observed at the interface of the mild steel substrate to the IMC layer and Al alloy substrate. With an increase in friction time, the volume of the IMC was increased and the Al oxide film remained in narrower areas to be almost disappeared at the maximum bond strength. At friction times shorter than that to obtain the maximum bond strength, fracture occurred mainly along the Al oxide film, suggesting that the bond strength was controlled by the Al oxide film. At friction times longer than that to obtain the maximum bond strength, joint were fractured in the IMC layer, which occupied almost whole areas of the interface. The bond strength of these joints decreased almost linearly with the increased in the width of the IMC layer irrespective of the friction time and pressure. Thus the factor controlling the bond strength can be considered to alter from the Al oxide film to the IMC layer with the increase in friction time. The IMC layer consisted mainly of Fe2Al5 and Fe4Al13. Small amount of FeAl2 was also detected. These compounds were granular and distributed randomly in the IMC layer. The width of the IMC layer was increased almost in proportion to the friction time at a rate which rose with friction pressure. These results suggest that the growth of the IMC layer is controlled by mechanical mixing of Al and Fe at the interface as well as the diffusion.