Abstract: The RPS (sample solidified under reduced pressure) was made of a sand sample cup with which the conventional preheated metallic sample cup was substituted.As a result, the hydrogen blow hole degree of aluminium alloy casting can be predicted by the RPS' density and the quality of the melt can be controlled. Moreover a prevacuumized chamber is designed in the test apparatus, and the solidification chamber may reach the reduced pressure value in the shortest time so that the more exact test can be done. The properly reduced pressure of solidification with sand cup is about 11.3 kpa.