X-ray diffraction (XRD) analysis with Rietveld refinement was used to determine the percentage of each microstructural constituent in AISI 5160 steel parts, which were austempered at 315°C for 1 minute, 2 min, 5 min, 30 min, and 90 min, separately. Nanoindentation was also conducted to quantify the concentration of bainite to compare with the XRD results. Thus, the bainitic transformation kinetics for AISI 5160 was determined for the selected austempering temperature. The mechanical properties of these parts were also tested to investigate their correlation to the bainite percentage. The results show that the tensile properties, including the yield strength, ultimate tensile strength, and elongation, increase with the austempering holding time at the selected austempering temperature, which corresponds to the bainite percentage increase. And in the same vein, hardness (both macro- and micro-hardness) decreases with austempering holding time, which corresponds to the martensite percentage decrease.