The effects of austenite grain size on the bending fatigue crack initiation and fatigue performance of gas carburized, modified 4320 steels were studied. The steels were identical in composition except for phosphorus concentration which ranged between 0.005 and 0.031 wt%. Following the carburizing cycle, specimens were subjected to single and triple reheat treatments of 820°C for 30 minutes to refine the austenite grain structure, and oil quenched and tempered at 150°C. Specimens subjected to bending fatigue were characterized by light metallography to determine microstructure and grain size, X-ray analysis for retained austenite and residual stress measurements, and scanning electron microscopy for examination of fatigue crack initiation and propagation. The surface austenite grain size ranged from 15 μm in the as-carburized condition to 6 and 4 μm diameter grain size for the single and triple reheat conditions, respectively. The reheated core grain structures were coarser than the reheated case grain diameters. Endurance limits for the reheat specimens were determined to be 1400 MPa for the triple reheat specimens, independent of phosphorus concentration, and 1350 and 1275 MPa for the single reheat specimens containing 0.005-0.017 and 0.031 wt% phosphorus, respectively. Grain size refinement caused a shift in bending fatigue crack initiation from intergranular cracking associated with phosphorus segregation and carbide formation to intergranular cracking limited to the intergranular oxidation zone of gas-carburized specimens.