not-yet-known not-yet-known not-yet-known unknown Steel is heat treated by various methods and in the past 10 years induction and furnace heating has been combined in order to improve fatigue strength. In this present work, we focus on furnace induction heating (FIH) and induction furnace heating (IFH) of SUJ2 steel. We performed rotating bending fatigue tests in order to clarify the effect of the FIH and IFH methods on fatigue strength. We analyzed the fracture surface in FIH samples based on Komotori’s fracture mode transition concept and investigated the improvement of fatigue strength of FIH and IFH samples. This improvement could not be explained by the following well-known factors: size of inclusion and FGA, hardness at crack origin, and stress at the crack origin. Therefore we propose a new way to explain the fatigue improvement in the FIH and IFH samples based on carbide accumulation near inclusions.