The metallographic factor controlling the strength of friction-welded interface of mild steel to Al-Mg alloy A5083 has been investigated by TEM observations. The bond strength, estimated from the tensile strength of a specimen with a circumferential notch at the interface, rose rapidly with an increase in friction time, and then reduced. A maximum strength of 311 MPa was obtained at a friction time t1 of 2 s (rotation speed = 20 s-1, friction pressure = 40 MPa, and forge pressure = 230 MPa). At a friction time of 1 s, an interfacial layer about 100 nm wide that consisted of (Fe,Mn)Al6 and Mg2Si was formed at the interface, and an Al oxide layer of width less than 10 nm was observed between the (Fe,Mn)Al6 layer and mild steel substrate. The joint bonded at t1=1 s was fractured mainly along the Al oxide layer. In a joint showing the highest bond strength (t1 = 2 s), no Al oxide layer could be detected between the mild steel substrate and interfacial layer, which consisted of (Fe,Mn)Al6, Fe4Al13, Fe2Al5, and Mg2Si. The width of the interfacial layer was increased to about 300 nm at t1=2 s. The fracture occurred along the IMC layer of (Fe,Mn)Al6, Fe4Al13 and Fe2Al5. At t1 of 4 s, a layer of MgAl2O4 about 100 nm in width was observed in addition to the intermetallic compounds observed at t1= 2-3 s. The crack on the tensile test was propagated mainly along the MgAl2O4 layer. Thus the controlling phase of the bond strength was altered from the Al oxide film, intermetallic compound layer, and MgAl2O4 layer, depending on friction time. The formation mechanisms of the observed interfacial phases are discussed from a metallographic point of view.