The solid-state friction welding method, which is widely used in the automotive, machinery, aviation and aerospace industry, was applied in this study. AISI 420 martensitic stainless steel, which is difficult to join by using fusion welding methods, and AISI 4340 tempered steel, which is commonly used in the automotive industry, were joined. During the welding process, three different rotations as 700, 1000 and 1300 rpm were used and parameters such as friction pressure, friction time, upset pressure as well as upsetting duration were kept constant. After the welding process, welded joints were examined macroscopically and microscopically, and their microhardness were measured. Then, torsion tests were performed on these samples. During the torsion tests, the samples fractured outside the welded joint regions. Thus, it was concluded that torsional strength of welded joint zones was high and AISI 420 and AISI 4340 steels were smoothly joined by friction welding.