Rotational bending fatigue tests were carried out on a medium carbon steel with a thermally sprayed Co-based alloy coating. The effect of two different fusing treatments on the fatigue strength of fused specimens was investigated. Fusing treatment was performed using a vacuum furnace, and an induction heating system. When the specimens were treated in vacuum furnace at 1373 K for 4 h, the diffusion layer providing a strong adhesive force were formed at the interface between the coating and substrate. As a result, fatigue strength remarkably increased in comparison with the uncoated specimens. On the contrary, for the treated specimens with an induction heating system at 1373 K for 120 s, since these specimens had a lower adhesive force due to no formation of diffusion layer, leading to delamination of the entire coating.