Gas porosity is one of the most common casting defect in die cast parts and it has an undeniable effect on strength, leakage and heat treatability. Reducing and even preventing porosity level can be achieved by the integrative design of mold, vacuum system and optimized process parameters. Applying semi-solid processing also helps to improve quality; not only by preventing defects but also modifying microstructure to have stronger products. In semi-solid casting, generated globular primary phases cause reduction in apparent viscosity under increased shear stress in thixotropic behaviour. This leads to planar flow instead of turbulent, which helps reducing air entrapment during injection and mold filling. In this study, GISS (Gas Induced Semi-Solid) adapted high pressure die casting technique is used for producing AlSi9Cu3 alloy die cast parts. The effects of semisolid processing temperature, rheo-casting time, changeover position and fast shot velocity are investigated on microstructural changes and porosity levels by using Taguchi method. Radiographic analysis is used for non-destructive testing for determining porosity state. Optic and field emission scanning electron microscope (FE-SEM) are used for microstructural and morphological characterization. Energy-dispersive X-ray spectroscopy (EDS) is also applied for chemical analyses of the phases involved. As results, fast shot velocity and rheo-casting time are found to be the most effective parameters on the porosity level and globular microstructure, by increased shear stress.