Fatigue tests were conducted on aluminum alloy (2024-T6) reinforced with 20 volume percent and two different sizes of SiC particles using a specially designed servo-hydraulic fatigue testing equipped machine in a scanning electron microscope. The influence of reinforcement-particle size on the initiation and propagation behavior of small fatigue cracks in a smooth specimen was examined. In the case of fine particle-reinforced composites, crack initiation was associated with matrix slip in a particle-rich region. When the crack propagated from the particle-rich region to a matrix-rich region, sub-crack and crack bridging by particles were observed. In the case of coarse particle reinforced composites, cracks were mainly initiated by particle cracking, and particle fracture ahead of a main crack and crack bridging by an uncracked ligament were observed.