The precision steel castings were produced with nitrogen（N2）or propane gas（C3H8）filling the shell mold before and after pouring liquid steel in case of sand-free.The microstructure was observed and the micro-hardness of the castings was measured on a type HXD-1000 micro-hardometer.The reason resulting in the relief of surface decarborization in the precision steel castings has been approached with protection gas.The results show that although both the N2 and C3H8 can effectively relieve or prevent surface decarborization in precision steel castings,C3H8 exhibits better prevention-decarborization effects.Surface decarborization in precision steel castings can be effectively eliminated as a result of removing the air in the shell mold.