Friction stir lap welding (FSLW) experiments have been conducted to study the effects of tool positioning on microstructures formed in the Al-to-steel interface region and on joint strength, defined as maximum applied force over the width (F (m)/w (s)) of the test sample, of the welds. Various pin positioning and speed conditions were used in the FSLW experiments followed by microstructure examination on the interface regions and tensile-shear testing on the welds, including an examination on crack propagation in mixed stir zone. It was found that when the pin was close to the bottom steel piece, Al-to-steel reaction occurred resulting in intermetallic outbursts formed along the interface. This represents the case of incomplete metallurgical joint. When the pin was lowered to just reach the steel, a thin and continued interface intermetallic layer formed. Evidences and consideration on growth kinetics have suggested that the layer could only remain thin (a parts per thousand currency sign2.5 mu m) during FSLW. This layer could bear a high load during tensile-shear testing and the adjacent aluminium deformed and fractured instead. The resulting F (m)/w (s) was high. When the pin penetrated to steel, F (m)/w (s) reduced due to brittle fracture being dominant inside mixed stir zone. Evidences have shown that the amount of penetration and speed condition during FSLW do not have large effects on F (m)/w (s).