An eruption of porosities which degrade strength reliability of products, is inevitable in the die casting process. We have investigated PRPC (Porosity Reducing Post Compression) process of ADC 12 aluminum alloy die castings to reduce inside porosity. In this study, a validity of PRPC process is demonstrated in terms of the Charpy impact strength. We ensure that the porosity ratio was considerably reduced by the PRPC process by means of X-ray CT observation. We conclude that the Charpy impact strength improves since the effective cross-sectional area of the specimen concerning fracture increases after PRPC process without any harmful influence. This conclusion is exemplified by the flat fracture surface of the specimen after PRPC process.