Many practical high pressure die casting process (HPDCP) optimization problems are multi-objective optimization ones that optimize multiple quality attributes of castings. This paper proposes a new HPDCP optimization method for improving shrinkage porosity and air entrainment using Taguchi-based ProCAST simulation and multi-attribute decision making (MADM)-based overall quality index. Taguchi orthogonal array is used to design ProCAST simulation experiment. MADM is used to convert multiple quality attributes into a single overall quality index (OQI). Taguchi optimization method is used to determine optimal HPDCP parameters to maximize the OQI. By using the proposed method, this paper determines the optimal HPDCP parameters such as pouring temperature (PT), filling rate (FR), piston velocity (PV) and preheating mold temperature (PMT) for improving volume of shrinkage porosity (VSP) and air entrainment (AE) in carburetor housing with aluminum alloy AlSi9Cu1Mg. The optimal HPDCP parameters are PT of 640 oC, FR of 40%, PV of 6.5 m/s, and PMT of 150 oC. The PT is the most effective HPDCP parameter for improving the VSP and AE, and the next are FR, PV and PMT. The proposed method could be actively applied to not only HPDCP but also other casting processes and other manufacturing processes.