Gas pores on the top face of engine crankcases which are produced by Low Pressure Die Casting (LPDC) have been studied. Micrographic analysis has been conducted to investigate the evolution of such kind of casting defects. Results show that the surface of such casting defects have meandering shapes and random sizes. Microstructures of the cross section of the defects displayed folded feather shapes, which can be produced by the folded oxide film into the melt during the filling process. Oxide inclusions have been found below the filter at the ingate area. Thus, it is confirmed that the folded oxide films came from turbulence of the metal in the die cavity. Numerical simulation has been carried out to study the filling behavior of the crankcase. Results show that gating velocity suddenly increased when the liquid metal got into the waterfall structure and led to the turbulence. Moreover, the air entrained behavior is related with the gating velocity. With increasing gating velocity, turbulence happened when metal confluences, and became more intensely in the middle of the casting due to its waterfall structure. Finally, this kind of casting defect was confirmed as gas entrainment.