The effect of silicon carbide (SiC) on small fatigue crack initiation and propagation behavior of A6061-T6 metal matrix composites reinforced with 20 vol. % or 30 vol. % SiC particles of various diameters was investigated under axial cyclic loading at the stress ratio R of 0.1. The results obtained are summarized as follows. (1) The fatigue cracks of these composites initiate preferentially at the sharp end of a SiC particle in the matrix aluminum material, because the stress near the sharp end of a particle appears to be much higher than the average stress. (2) Small cracks exhibited much higher growth rates just after crack initiation, followed by a decrease in growth rate to a minimum value, which corresponded to that of a Stage I crack, and a monotonic increase with increasing ΔK. (3) The highest fatigue strength of the 20 vol. % SiCp (mean diameter is 7 μm) composite was due to the excellent small crack growth resistance of this material.