Abstract: This paper investigates the effects of microstructures to the initiation and propagation of cracks in SiCparticle Al composite materials by optical microscope and scanning electron microscope,10% and 20% volume percentages SiC_p/Al composite are usedResults show that the 10% volume percentage SiC_p/Al is more sensitive to the micro-notch than the 20%one. The cracking and propagating behavior are not only affected by themacroscopic remote stresses, but also controlled by microstructures ahead of the crack. SiC particle defects can either prohibit crack propagation,or change the cracking path.Microstructural defects can increase crack growth rates,The propagation is non-uniform.