We report expendable pattern casting(EPC) of AZ91D Mg alloy, which has been well known as a high strength lightweight material. The stairway patterns were set at the top gating system to evaluate the mold filling velocity in the self-designed EPC system equipped with atmosphere, pressure and vacuum control apparatus. To measure the passing time of the liquid Mg alloy penetrating into the foam pattern prepared, we have inserted steel wires at each of desirable position in the pattern. It is observed that the average penetrating velocity of the liquid Mg is dependent on not only coating materials, which were varied by permeability but also the degree of vacuum. The convex-type shape of metal front, which was shown at a condition without assistance of vacuum, is changed to the concave type shape, which was shown under vacuum of -50 mmHg.