The effect of grain-boundary reaction precipitates on the high-temperature fatigue crack growth was investigated using austenitic 21 Cr-4Ni-9Mn steel at 937K in air. Grain boundaries were serrated by the grain-boundary reaction precipitates. The crack-growth rate was considerably decreased by these precipitates, especially under the condition of a low crack-growth rate. The fatigue cracks extended on the serrated grain boundary or on the interface between the grain-boundary reaction nodule and the grain. Therefore, the cracks grew on the zigzag paths, and brittle intergranular fracture was then retarded. The retardation of the fatigue crack growth was explained by these changes in fracture mode.