The effects of microstructural characteristics (secondary dendrite arm spacing, SDAS) and Si- and Fe-based eutectic structures on the mechanical properties and failure behavior of an Al-Si-Cu alloy are investigated. Cast Al alloy samples are produced by heated mold continuous casting (HMC) process. Dendrite cells appear to grow in the casting direction. SDAS decreased with increasing casting speed .There are linear correlations between casting speeds and tensile properties (ultimate tensile strength crurs and fracture strain ef). These linear correlations, however, break down, especially for ours versus casting speeds and ef versus casting speeds. These results cause a-Al phase and eutectic phase (Si- and Fe-based). These microstructural effects on tensile properties are explained using a proposed model.