Fatigue behavior and mechanical properties of CK45 mild carbon steel has been investigated under combined heat treatments processes. The 80 standard samples were machined, quenched and tempered at various temperatures for 20 min, then subjected to several tests. The results shows that the fatigues limit of treated samples are more than non-treated samples significantly. The fatigue strength of treated samples at 450 C is approximately 2 times more than non-treated samples. In addition, the fatigue limit does not increase with temperature and it has pick point, in other words further heating above 450 C does not increase the fatigue strength significantly. This increasing in fatigue strength is due to the dual phase microstructure which is changed by treatment processes. Configuration of the dualphase microstructure can delay crack initiation and propagation processes.