Cooling conditions and inclusions are of the most important factors that affect mechanical properties of cast aluminium alloys. This study investigated the effect of secondary dendrite arm spacing (SDAS) on the shape of pores (i. e. bifilm unravelling) and the mechanical properties of cast A356 alloy. Different cooling conditions were established by electrolytic copper chill, H13 steel chill and insulated ceramic that was placed in the mould cavity. SDAS and shape of pores were investigated by optical microscopy. The fracture surfaces of tensile test samples were analysed by SEM and EDX. Weibull two-parameter statistical method was used to assess the tensile properties. Results show that mechanical properties were dominantly affected by pore morphology that was formed by bifilms. Increasing the cooling rate (i. e. decreasing SDAS), the unravelling of bifilms delayed which decreased porosity formation significantly.