Abstract: The isotopic abundence ratios for barium at sub-microgram level are analyzed by thermal surface ionization mass spectrometry (TIMS). Rhenium trips used for sample preparation are firstly treated to (eliminate) possible harium background interference. During the preparation of barium samples phosphoric acid is added as an emitting and stabilizing reagent. The addition of phosphoric acid increases the collcction efficiency and ion current strength and stability for barium. A relative standard deviation of (0.02%) for the isotopic abundence ratio of~(137)Ba to~(138)Ba is achieved when the~(138)Ba ion current is (1～3)×10~(-12)A. The experimental results also demonstrate that the isotope fractionation effect is negligibly small in the isotopic analysis of harium.