Cast Al-Si alloys have been widely used in automotive applications with regard to their low density and excellent thermal conductivity. Many components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. Furthermore, for these materials the well know size effect in fatigue, whereby the fatigue strength is reduced when the size is increased, can be significant and need to be properly evaluated. This paper analyses the role of casting defects on the fatigue strength’s size effect sensitivity. A uniaxial fatigue testing campaign (R=0.1) has been conducted using two cast aluminium alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with the T7 heat treatment, and with different degrees of porosity. The fatigue response of different specimens (smooth and notched) with different stressed volumes has been investigated. The first part of this article is dedicated to the experimental characterization of the size effect in both alloys via the concept of the Highly Stressed Volume. The second part investigates the effect of the Highly Stressed Volume on the critical defect size via Kitagawa-Takahashi diagrams. The results show that the magnitude of the size effect and the experimental scatter are strongly linked to the characteristics of the defect population present in the alloy. It is revealed that the alloy B, with a high density of pore and a population of defects with relatively large size, shows non-significant size effect and less scatter in fatigue strength. In comparison, alloy A that exhibits a low density of pore and a population of defects of relatively small size manifests significant size effect and high scatter in fatigue strength.Read less <