The most common defect found in high pressure die casting (HPDC) process is the gas porosity which significantly affects the mechanical roperties of the final components. The generation of gas porosity is known mainly due to the air entrapment in the liquid metal during the mold filling stage. Knowing the trapped–air location and amount could allow for a more accurate and objective analysis of casting quality. In the past few decades, extensive efforts have been made to develop simulation codes of casting flow. Most of these codes solve the velocity, pressure and fluid fraction only in the liqid phase wih he assumption that the effect of air in the die cavity is negligible. As a matter of fact, the air in the die cavity has significant influence on the filling pattern of the molten metal and the gas porosity distribution of the die casts. Recently, following the development of computational fluid dynamics (CFD), two–phase flow models have drawn continuous attention in the numerical simulation of casting processes, but there are still few models and further studies are needed. In this study, the mechanism of the formation of air entrapment defects in the HPDC process was discussed and it turned out that the air flow in the die cavity as well as the interaction between air and liquid metal resulted in the final air entrapment. In order to consider the air flow in the die cavity, an incompressible two–phase flow model was presented to simulate the air bubbles entrapped by the free surface of liquid metal durig the mold filling stage. Two numerical benchmark tests of fluid dynamics were performed to verify thvalidity and stability of the model. Furthermore, a high speed water analog exeriment similar to real die casting process was designed and carried out to compare the exerimental results with the simulation ones. Good agreements obtained demonstrate that the two–phase flow model has acceptable rediction accuracy in modeling the filling behavior of liquid and tracking the entrapped air bubbles.