The influence of microstructures on the growth behavior of small cracks is investigated using three kinds of heat-treated squeeze-cast aluminum alloys. A eutectic silicon particle becomes the initiation site for the crack and has a blocking effect upon crack growth. Although the growth behavior of the small crack is strongly influenced by microstructures in the early stage of crack propagation, the influence of microstructures decreases with increase in the crack length. The critical length at which the marked influence of microstructures is observed is about 200μm, which is nearly 8 times the dendrite arm spacing, regardless of heat treatment. Mean values of the crack growth rate in the growth process affected by microstructures, however, may be extrapolated from the crack growth rate in the range where the growth behavior is mainly controlled by a mechanical parameter alone.