For the prediction of air-entrapment in aluminum die castings, we proposed the Critical Point Searching Method (CPM) : a technique for searching for points where the velocity vector is zero. The positions of the critical points can be found by searching all cells in the flow field. The flow field was calculated using a casting flow simulator Stefan 3D, developed by Tohoku University. The tetrahedral-cell based approach was used to calculate steadily and efficiently. Three examples of fluid flow simulation were conducted. Close relations were found between the simulated results and the results of X-ray penetration examinations and CT pictures of castings.