The fatigue crack growth behavior of powder metallurgy steels (P/M steels) is strongly affected by the nature of porosity and microstructure of steel matrix. Our previous work has focused on a premix P/M steel prepared from Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6% graphite. In this study, we have studied the fatigue crack growth behavior of a prealloy steel of similar composition. Use of the prealloy powder resulted in more homogenous microstructure than the premix steel. The alloys were tested at three different densities: 7.0 g/cm3, 7.3 g/cm3, and 7.5 g/cm3. Microstructure characterization was conducted by optical and scanning electron microscopy (SEM). Fatigue testing was performed at various R-ratios, ranging from -2 to 0.8. Prealloy steels had a higher fatigue resistance than premix steels. Increasing porosity and increasing R-ratio resulted in a decrease in ΔKth. The degree of crack closure was measured for both premix and prealloy steels at different R-ratios, and is discussed.