Abstract: T2 copper and Q235 steel dissimilar metals were friction stir lap welded (FSLWed) using the stir tool made by SKD61 die steel. The microstructure and mechanical properties of FSLWed joints were studied. The results showed that the stirring on the Q235 steel of the stir tool leaded to constant wear even fractured. There was stream-lined structure in the advancing side of nugget zone and mechanical and metallurgical bonding was formed in interface zone (IZ). The micro-hardness test showed that the micro-hardness of weld nugget zone (WNZ) was the highest in copper side. The micro-hardness of interface zone (IZ) in weld center was higher than that of the both sides, which meaned that the micro-hardness of IZ was the highest along the thickness direction. The defect-free joint fractured in the heat affect zone (HAZ) of copper side in the tensile-shear test. The fracture surface morphology of lap joints was characterized by dimples, showing that it was ductile fracture.