Fracture mechanics approaches to life prediction are being considered for applications to highly stressed components operating at high frequencies. One such application is rotor hubs on helicopters. For such applications, it is imperative that the growth behavior of small cracks be well understood. This study intended to examine the effect of material microstructure on the small crack growth behavior of PH13-8Mo stainless steel under both constant and variable amplitude loading. Constant amplitude loading at six different stress amplitudes showed that the crack growth rate did not depend upon the stress amplitude. The small crack growth behavior was greatly affected by the size of the martensite packet. Furthermore, it was observed that the small crack growth rates were a little slower than long crack data. Single overload tests indicated there was inconsistent crack growth behavior (crack growth acceleration and retardation were both observed after overload). It was suggested that this inconsistent crack growth resulted from the crack tip location relative to the microstructural features at the time of overload application. However, the simple block loading showed that there was overall crack growth retardation. As expected, the extent of the crack growth retardation increased with increasing the maximum stress of the overload stress block.