In this study, tailor-welded blanks (TWBs) composed of A5052P-O aluminum and AZ31B-O magnesium alloys were produced by the friction stir welding process. The sound surfaces without large defects were successfully obtained at the tool rotation speeds ranging from 1000 to 1400 rpm under the constant tool traverse speed of 300 mm/min, and the surface roughness was decreased in the friction-stir-welded zone (SZ) with the increase in the tool rotation speed. Also for 100, 300 and 500 mm/min, the sound surfaces without large defects were obtained under the constant tool rotation speed of 1400 rpm. The increase of the tool traverse speed led to the decrease in the surface roughness of the SZ. In the SZ, the bonded interface was clearly evident with plastic flow pattern between the aluminum and magnesium alloys, although an onion ring pattern was not formed. During tensile testing, the TWBs were fractured in the SZ at the early stage of the plastic deformation, i.e. strain hardening region, without remarkable area reduction near the fracture region. The TWBs exhibited the similar average tensile strength, showing that there were not noteworthy changes in the tensile strength as a function of the tool rotation speed. The average tensile strength, however, was slightly decreased with the increase in the tool traverse speed. It is noticeable that, in all cases, the joint efficiency of the TWBs exceeded 62%, and the maximum average tensile strength of about 143 MPa was obtained at 1400 rpm under the tool traverse speed of 100 mm/min, which was nearly equivalent to the joint efficiency of about 72%. The average total elongation of the TWBs was about 2% or less considerably lower than those of the base metals, without significant changes as functions of the tool traverse speed and the tool traverse speed.