Fatigue crack growth (FCG) behavior has been investigated for two different grades of austempered ductile irons (ADIs). These ADIs were produced from an alloyed ductile iron (DI) and heat treated respectively at two austempering temperatures, 300 and 360degreesC, to generate two different ausferrite microstructures. FCG tests using compact tension (CT) specimens were conducted under load control with three load ratios, R = 0.1, 0.5 and 0.7. The fatigue crack growth rates (FCGRs) of the given ADIs were compared with those of the as-cast DI with a bull's eye microstructure to examine the influence of austempering treatment on the FCG behavior of DI. The FCG behavior for the given materials was found to be dependent on the matrix structure with a demonstration that the as-cast DI had a better FCG resistance than did the ADIs at low DeltaK regime and vice versa at high DeltaK regime. As for the comparison made between the two ADIs, the one austempered at 360degreesC exhibited a lower FCG rate as a result of its coarse ausferrite microstructure, higher volume fraction of retained austenite, and greater toughness. The ADIs also demonstrated a load ratio dependence of intrinsic FCGR; that is, the enhancement of the FCGR with an increase in R value could not be rationalized by the crack closure effects.