Abstract: Rare earth fluorides were synthesized with RE₂O₃ and NH₄HF₂ under atmospheric pressure and vacuum in RE₂O₃-NH₄HF₂ systems, the reactions in the systems were studied, and a new mechanism for the synthesis of REF₃ was proposed. Powder La₂O₃, Gd₂O₃ started to react with NH₄HF₂ at low temperature, and the products were RENH₄F₄(RE=La, Gd), NH₄F, NH₃ and H₂O. RENH₄F₄ was decomposed to REF₃ and NH₄F at high temperature, accompanied with the volatilization and decomposition of NH₄F. The whole process could be divided into three steps of synthesis, decomposition and deammoniation. The initial and final temperatures of the reactions were reduced under vacuum. The combination process of synthesis under atmospheric pressure at low temperature and decomposition and deammoniation under vacuum at high temperature should be applied to the preparation of rare earth fluorides.