In recent years, demand for high thermal conducting heat-sink products, generally produced with the die casting process is increasing. For an ADC12 alloy mainly used in industry, its thermal conductivity decreases around 18% compared to that produced by the gravity casting method. Herein, we produce the specimens with the variation of thickness (that is, cooling rate), to understand the effect of the microstructure on thermal conductivity. Microstructures are analyzed by optical microscope (OM) and scanning electron microscopy (SEM). And thermal properties are analyzed by laser flash analysis (LFA) and differential scanning calorimetry (DSC). For the ADC12 alloy containing high silicon, as the cooling rate increases, microstructural defect of pores is observed to be increased, leading to low thermal conductivity. Thus, many nucleation sites of the alpha-matrix are required to reduce the defects in the alloy, which will be presented.