A multi-scaled model for the fracture toughness of an aluminum alloy has been developed that agrees well with the experimental data. It has been shown that fracture toughness decreases as the volume fraction of the first-class (related to microconstituents/ inclusions) and second-class (related to second phase precipitates) microcracks increase, with the first-class microcracks have greater and more obvious effects. Thus, decreasing the volume fraction of the micro-constituents (normally Fe and Si impurities) can decrease the volume fraction of the first-class microcracks and help improve the fracture toughness. At the same time, decreasing the volume fraction of the precipitates along the grain boundaries can also improve the fracture toughness by decreasing the volume fraction of the second-class microcracks.