The tungsten ore and molybdenum ore are refractory and usually dissolved by alkali fusion method. However, a lot of sodium ions in the flux would be introduced into sample solution and lead to high salinity with the pretreatment by alkali fusion. The high salinity could cause matrix interference and cone-hole blocking, which is disadvantageous to determination of trace rare earth elements by inductively coupled plasma mass spectrometry (ICP-MS).In order to solve this problem, the sample was fused with NaOH and Na₂O₂ at high temperature followed by leaching and filtration with hot water. Then the rare earth elements were enriched into rare earth hydroxide precipitates, realizing the separation from much sodium and other metallic ions such as potassium, tungsten and molybdenum. The precipitate was dissolved with tartaric acid-HCl system. After dilution, the content of rare earth elements in tungsten ore and molybdenum ore was determined by ICP-MS. The results showed that 0.500 0 g of sample could be fully decomposed with 3.0 g of NaOH and 1.5 g of Na₂O₂ in muffle furnace at 700 ℃ for 20 min. The mass spectrometry interference could be eliminated by selecting proper isotopes and mathematic correction methods. The analytical signal drift and matrix effect could be effectively monitored and corrected with 10 μg/L ¹⁰³Rh as internal standard. Under the optimal conditions, the correlation coefficients of calibration curves of all elements were all higher than 0.999 5. The detection limit of method was 0.004-0.08 μg/g. The experimental method was applied to the determination of rare earth elements in component analysis certified reference materials of molybdenum ore and tungsten ore. The absolute values of logarithm error (ΔlgC) between the results and the certified values were all less than 0.1 (the requirements according to industrial standard of DZ/T 0130—2006 for geology and mineral resources). The relative standard deviations (RSD, n=6) were all less than 5%.