The micromechanics of naturally initiated small fatigue cracks in coarse- and fine-grained nickelbased ASTROLOY have been studied at ambient and elevated temperature using a cyclic loading stage for the scanning electron microscope (SEM) and the stereoimaging technique. The objective of these experiments was to obtain a fracture mechanics description of crack driving force for use in engineering damage tolerance assessments. Cracks were initiated from small pores and slip lines within grains at ambient temperature in bending but were grown in tension at 20 °C and 600 °C. When expressed in terms of the linear elastic stress intensity factor range, ΔK, small cracks grew faster and at lower ΔK than did large cracks, as has been found for many other materials. Displacements were measured around the tips of small cracks, from which both crack opening displacements (CODs), and strains were derived. Mixed mode CODs were typically found. Because the small cracks exhibited large levels of crack-tip plasticity, an equivalent stress intensity factor (a local crack driving force based on ΔJ) was computed using measured crack-tip parameters. Crack opening load, measured by direct observation of the crack peeling open under high resolution conditions, was used to calculate an effective stress intensity factor. The relationships between the applied ΔK, as computed by standard fracture mechanics methods, the equivalent stress intensity factor, and the effective stress intensity factor for small cracks were found to be different than for large cracks. Crack growth rates for small and large fatigue cracks were correlated through the use of equivalent stress intensity factors.