A series of Al-Si-Mg-Cu alloys of different Si contents (namely 5 %, 8 % and 11 %) and with and without Na-modification were cast and heat-treated for 8 and 24 hours. Microstructures of these alloys were revealed and after the determination of the tensile properties, the fracture surfaces were examined. It has been observed that the crack follows the Si phase in these alloys and therefore, the fracture toughness is not affected by the Si content. However, altering the morphology of the Si phase (i.e. spheroidising) affects the fracture toughness greatly; as the Si phase gets circular the fracture toughness increases, where the highest toughness is attained in the Na-modified alloys for all compositions.