In automotive industry, many of the powertrain components (for e.g. engine head and cylinder block) are generally manufactured by a casting procedure. Secondary Dendrite Arm Spacing (SDAS) is one of the most important microstructural features in dendritic solidification of alloys (for e.g. Al-Si alloys) during the casting process. SDAS has a significant influence on the mechanical behavior of the cast aluminum components. A lower value of SDAS is desired in order to achieve better fatigue strength of the cast components which can be controlled by governing several casting parameters. For directional solidification, SDAS is dependent on various casting parameters i.e. chemical composition of the alloy, cooling rate and liquid melt treatment. During industrial casting of an alloy with predefined chemical composition, cooling rate during the mushy zone becomes the dominant parameter for controlling SDAS. The objective of this study was to predict the SDAS of die cast Al-Si alloy samples subjected to different cooling rates by varying the mold temperature. The SDAS was predicted by a casting simulation and utilizing the empirical relationship between solidification time and SDAS. The predicted SDAS values from simulation were compared with the experimental results of SDAS obtained from the microstructural inspection of the cast samples. It was observed that the predicted SDAS were in good agreement with the inspected values. This study also showed an increasing trend of SDAS values with the increase in solidification time. The approach to calculate SDAS, described in this paper could be used as a design tool to compare the fatigue properties of cast aluminum components subjected to different casting parameters without any destructive fatigue failure testing.