Titanium and mild steel sheets were welded by resistance spot welding with insert of 100 μm thick niobium foil. The interfacial microstructure characteristics were analyzed; the effects of welding on the nugget diameter and tensile shear load of joint were studied. The results reveal that the niobium foil disconnect during spot welding when the welding current is greater than 8 kA, the nugget is mainly composed of Fe-Ti intermetallic compounds in the case; and that the nugget is mainly composed of Fe-Nb compounds and some solid solution when the niobium foil do not disconnect during spot welding. The tensile shear load of joint increased and then decreased with the increase of welding current. A tensile shear load of a maximum of 4.7 kN was obtained at a welding current of 7 kA.