Threshold and fatigue crack propagation results were observed for a high strength low alloy steel at temperatures ranging from 123 to 300 K and grain sizes ranging from 10 to 123 μm. The threshold stress intensity was observed to increase as the grain size increased. Also a substantial increase in threshold was exhibited as the test temperatures were reduced from 300 to 123 K. The mode of fracture near and at threshold was ductile for all temperatures and grain sizes. However, at intermediate ΔK levels and low temperatures the mode of crack propagation was brittle. Superimposed on the river lines of the cleaved fracture surface were cyclic cleavage striations indicative of the local fracture mode. The fatigue crack propagation exponent n of the Paris law increased rather drastically as the temperature decreased and was determined to be directly associated with the brittle fracture process. Emphasis was placed on the subgrain cell structure that develops during cyclic deformation as being a very likely microstructural unit controlling the fatigue threshold process. Thus the grain size influence of the threshold is partially interpreted in terms of the subgrain cell size dependence on grain size. The temperature effect on the threshold is discussed from a thermal activation analysis view of dislocation behavior and plastic flow at low temperatures.