Abstract: Sample of RE-Mg-Si-Fe alloy was dissolved with con. HF and con. HNO3, and Mg2+ in the solution was determined by FAAS. La(Ⅲ) solution as releasing agent was added to eliminate the interferences of coexisting ions. Linear relationship between values of absorbance and mass concentration of Mg2+ was kept in the range of 0.2-12.0 mg·L-1, and detection limit (3s) of the method found was 0.057 mg L-1. The proposed method was applied to the analysis of two samples, giving values of recovery found by standard addition method of 99.4% and 97.6%, and RSD′s (n=11) of 0.89% and 0.43% respectively.