Fatigue strength and fracture mechanism of a medium carbon steel having carbon content of 0.35%C with gas flame thermally sprayed Co-based alloy coatings were investigated by rotating bending tests. After fusing treatment, machining was done to make three kinds of specimens having 0.3mm, 0.5mm and 1.0mm in coating thickness. The fatigue strength of all coated specimens was much higher than that of the substrate and that of the grit blasted ones. Especially, it was found that the fatigue strength of the specimens with 1.0mm coating was remarkably higher compared to those with 0.3mm and 0.5mm coatings. That is, fatigue characteristics depend on coating thickness, and fatigue strength increases with the increase of coating thickness. The result showed that at lower stress levels, the fatigue cracks were initiated only inside the substrate of 0.3mm and 0.5mm coated specimens (Internal failure in the substrate). At higher stress levels the fatigue cracks were initiated at the coating layer, originated from the porosity located in coating layer of all coated specimens (Coated layer failure). But no fracture mode transition was noticed for the case of 1.0mm coated specimen. The cause of the fracture mode transition depending on the coating thickness was discussed on the basis of the results from calculation of the stress at the location where the fatigue fracture was initiated.