This paper described an experimental study on the friction stir welding of dissimilar metal between commercially pure titanium (CPTi) and magnesium alloy AZ31B (Mg). Butt-welds produced at various tool rotation speeds were evaluated by the observation of the weld surface appearance, X-ray radiographic test, tensile test, SEM and EDX analysis. The main results obtained are as follows. Butt-joint welding of the CPTi plate to the Mg plate was easily and successfully achieved. The ignition of Mg occurred during welding at the tool rotation speeds over 1200rpm. The fragments of CPTi existed in a continuous form in Mg matrix. The tool rotation speed of 1200rpm attained the maximum tensile strength of a joint, which was about 75% of that of Mg base metal. Fracture occurred at the weld interface in most joint. EDX analysis revealed that Al in the Mg diffused into CPTi through the weld interface and MgO existed at the interface. It seems that the decrease in Al concentration in the Mg and MgO formation around the weld interface caused the low tensile strength of the joints.