Abstract: Pure titanium and mild steel Q235 sheets were welded by resistance spot welding. The nugget microstructure was observed and analyzed by using scanning electron microscopy. The effects of welding current on nugget size and tensile shear load of the joint were investigated. With the increasing of welding current, the nugget diameter increased while the tensile shear load of joint increased and then decreased when welding current was over 8 kA. A tensile shear load of a maximum of 2.85 kN was obtained at a welding current of 8 kA. The nugget of steel side was mainly composed of a 30～50 μm thick TiFe2+α-Fe eutectic structure layer adjacent steel and coarser TiFe columnar crystals. An approximately 12 μm thick TiFe+α-Ti eutectic structure layer adjacent titanium and TiFe columnar crystals were observed in the nugget zone of titanium side where macro stratification was also observed.