Vibrational uniaxial fatigue measurements are reported for an austempered ductile iron containing 3.4 wt.% C, 2.78 wt.% Si, 0.44 wt.% Cu, 0.61 wt.% Ni, 0.2 wt.% Mo, and 0.21 wt.% Mn. The iron was austenitized at 875 degreesC and then austempered at 260, 285, 320, 365 and 400degreesC for 1.5 hours to obtain various austempered microstructures. X-Ray diffraction was used to measure the amount of high carbon austenite and its carbon content. It is shown that fatigue strength increases as the amount of high carbon austenite increases. The highest fatigue strength is obtained with an ausferrite structure which has austempered at 365degreesC. This behavior is related to the volume of high carbon austenite and ferrite needle width. Fatigue cracks are arrested by ferrite needle boundaries. By increasing the austempering temperature the austenite volume increases and the possibility of crack propagation decreases from the surface of samples. This is related to the TRIP mechanism that produces martensite on the surface of samples. The ferrite needle width increases as the austempering temperature increases. These two effects change the fatigue limit in opposite directions. This behavior will create a situation that an optimum would be observed at an intermediate austempering temperature.