The chemical element and its valence at the fracture surface of SiCp/Al2O3-Al composites synthesized by oxidative infiltration of Al melt were analyzed quantitatively using X-ray photoelectron spectrometer(XPS), the percentage content of various phase at fracture surface was determined accordingly. Additionally, the volume fraction of different phase in the composites was meassured by optical metallographic examination of the three-dimensional section. And then, by the comparison of phase content between fracture surface and section of the composites, the preference for the crack penetrating various phases of the composites was identified ,which in turn micro-fracture mechanisms of the composites were revealed quantitatively. It is proved that SiC particle size has a critical influence on the percent content of co-continuous Al2O3 and Al phases, as well as the micro-fracture mechanisms of this kind of composites. Based on the analysis of micro-fracture mechanisms, the moderate size (about 10μm) of SiC particulate would be beneficial to the mechanical properties of composites, the tested results of mechanical properties under room and elevated temperature verified the hypothesis.