When friction welded material is actually used for parts of a machine, its fatigue strength is an important problem. Especially, there is no report which deals with mechanical properties of friction welded mild steel(S20C) and stainless steel(SUS27B). In this study are compared the compared the characteristics of such specimens as mild steel, stainless steel and welded material in the S-N diagram. And metallurgical consideration is directed to HAZ. The obtained results in these studies are summarized as follows; 1) The fatigue strength of welded material is slightly less than those of mild steel and stainless steel. 2) In the S-N diagram the knuckle point of welded material has larger number of cycle than that of stainless steel. 3) The fatigue notch factor of welded material is between those of mild steel and stainless steel. 4) mHv is the largest on the weld interface. It is larger before than after fatigue test on the stainless steel side. On the mild steel side it is the way around.