High-cycle fatigue (HCF) properties of a number of different grades of austempered ductile irons (ADIs) were investigated. Ductile irons were prepared as Y-block castings with three section sizes (25, 50, and 100 mm in thickness). Fatigue specimens were selected from various locations within the castings, austenitized at 1173 K, and then austempered at 573 and 633 K, respectively. HCF tests were conducted by rotary bending method to generate the S–N curves and the associated fatigue limits. As section size increased, the HCF strength of ADI was decreased and affected by the inferior graphite nodule morphology and microshrinkage porosity resulting from the slower solidification rate. The effects of section size, location within the heavy-section casting, and austempering temperatures on the HCF strength of ADI are interpreted in terms of the graphite nodule morphology and microshrinkage pores. An empirical equation was introduced to correlate the fatigue limit of ADI with the impact toughness value and mean nodule diameter.