Abstract: Effect of SiC particle(SiC_P) gradation (45 μm and (45+100) μm) on the deformation and damage of 70vol% SiC_P/Al composites was studied by an in situ X-ray imaging system. The strain fields of SiC_P/Al composites at different deformation stages under quasi-static loading were calculated by the X-ray digital image correlation method (XDIC). The bulk-scale stress-strain curves show that the difference in the yield strength of SiC_P/Al composites with different particle gradations is small. But the ductility of 45 μm SiC_P/Al is higher than that of (45+100) μm SiC_P/Al. Strain field mapping suggests that deformation and damage localizations appear earlier in (45+100) μm SiC_P/Al than in 45 μm SiC_P/Al, and the strain fields for (45+100) μm SiC_P/Al are more heterogeneous in the later deformation stages. The reason is that strain localizations tend to nucleate around big particles which grow and coalesce to form macroscopic cracks, leading to an earlier failure of (100+45) μm SiC_P/Al. Hence, optimizing the particle size distribution helps improve the ductility of particle-reinforced metal matrix composites. The postmortem analysis indicates that the fracture modes of the two SiC_P/Al composites are both brittle, characterized by particle breakage and interfacial debonding in the fracture plane.