Dissimilar 6061Al alloy/DP590 dual phase steel joints were fabricated using friction stir spot welding. The influence of rotation speed and dwell time on the feasibility of sound joints were examined by microstructural and mechanical evaluations. Microstructural examinations by optical and scanning electron microscopy exhibited fine grains structure in the stir zone of aluminum and dominant ultrafine ferrite grains with mean grain size of less than a micron and minor martensite at ferrite grain boundaries in the steel side. In addition, an intermetallic component layer was produced along the joint interface; its thickness was a function of process parameters. The tensile-shear strength of the joint was initially found to increase with increasing of tool rotation speed at constant dwell time and then decrease with further increasing. Same trend was observed with increasing of dwell time at constant tool rotation speed. Tensile-shear strength is influenced by the thickness of the intermetallic layer. A maximum tensile-shear strength of 205MPa
was achieved for the joint produced using tool rotation and dwell time of 2000rpm and 4s, respectively; this is higher than the shear yield strength of 6061-T6 Al.