The Gibbs free energy changes of the reactions of magnesium and its alloy with some materials were calculated at high temperature. The possibilities of the reactions were analyzed by free energy criterion .The results show that SiC and ZrB2 have good stability to liquid magnesium and its alloy and they are the good materials of crucible used for magnesium and its alloy melting. The stability of SiC in the liquid magnesium and its alloy was tested by experiment.