The effect of parameters of friction stir welding (the friction force, the forging force, the stir frequency, and the upset) on the strength of friction stir welded joints of unlike steels 30KhGSA and 40KhMFA is studied by the method of mathematical design of experiment. An empirical equation describing the dependence of the ultimate strength of notched specimens in the contact zone on the welding parameters is obtained. The range of optimum parameters providing maximum strength of the welded joints is determined. The mechanism of formation of a permanent joint, the microstructure, the distribution of microhardness and the mechanical properties in static tension tests are analyzed.