The primary objective of this study has been to investigate the fatigue behavior of the austempered unalloyed ductile iron. Ductile iron specimens were austenized at 900 degrees C for 110 min and then one group specimens were austempered at 230, 330, 430 degrees C for 60 min, and another one group specimens were austempered at 330 degrees C for 30, 60, 120 min to obtain different micro structures. Rotating bending tests (10 million cycles) were conducted on the ductile iron and the austempered specimens selecting completely reversed cycle of stress (R = minimum stress/maximum stress = - 1). The highest fatigue strength was found in the bainitic structure (ausferritic). The presence of martensite or carbide phases in the structure of austempered ductile iron reduces the fatigue strength. (c) 2005 Elsevier Ltd. All rights reserved.