Fatigue crack propagation (FCP) behaviors were studied to understand the role of SiC particles in 10 wt pct SiCp/A2024 composites and Si particles in casting aluminum alloy A356. The results show that a few particles appeared on the fracture surfaces in SiCp/Al composites even at high AK region, which indicates that cracks propagated predominantly within the matrix avoiding SiC particles due to the high strength of the particles and the strong particle/matrix interface. In casting aluminum alloy, Si particle debonding was more prominent. Compared with SiCp/Al composite, the casting aluminum alloy exhibited lower FCP rates, but had a slight steeper slope in the Paris region. Crack deflection and branching were found to be more remarkable in the casting aluminum alloy than that in the SiCp/Al composites, which may be contributed to higher FCP resistance in casting aluminum alloy. Fatigue crack propagation (FCP) behaviors were studied to understand the role of SiC particles in 10 wt pct SiCp / A2024 composites and Si particles in casting aluminum alloy A356. The results show that a few particles were found on the fracture surfaces in SiCp / Al composites even at high AK region, which indicates that cracks propagated predominantly within the matrix avoiding SiC particles due to the high strength of the particles and the strong particle / matrix interface. Compared with SiCp / Al composite, the casting aluminum alloy exhibition lower FCP rates, but had a slight steeper slope in the Paris region. Crack deflection and branching were found to be more remarkable in the casting aluminum alloy than that in SiCp / Al composites, which may be contributed to higher FCP resistance in casting aluminum alloy.