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The isothermal section of the phase diagram of the Tb-Fe-Ga ternary system at 773 K was investigated by X-ray powder diffraction analysis．Fourteen binary compounds,TbFe2，TbFe3，Tb6Fe23，a-Tb2Fe17,Tb5Ga3，Tb3Ga2，TbGa,Tb3Ga5，ThGa2，a-TbGa3，a-Fe3Ga,a-Fe6Ga5，Fe3Ga4 and FeGa3，have been confirmed and six ternary compounds，TbFe0.32Ga1.68，TbFe2.4Ga0.6，Tb4FeGa12，Tb2FeGa8，TbFe6Ga6 and ThFe5Ga7，were found in this ternary system at 773 K．The structures of theses compounds were determined by Rietveld refinement method．The compound TbFe0.32Ga1.68 is InNi2-type structure(space group P63/mmc)with a=0.43727(2)and C=0.72930(4)nm．The compound TbFe2.4Ga0.6 belongs to CeNi3-type structure(space group P63/mmc)with a-0.51877(9)and C=1.6621(4)nm．The compound Tb4FeGa12 is U4Re7Si6-type structure(space group Im 3m)with a=0.85576(4)nm．The compound Tb2FeGas is H02CoGa8-type structure(space group P4mmm)with a=0.4259(4)and C=1.1 1 356(8)nm．The compound TbFe6Ga6 is ScFe6Ga6-type structure(space group Immm)with a=0.85551(4)，b=0.86826(5)and C=0.50717(3)nm．The compound TbFejGa7 is ThMn12-type structure(space group 14/mmm)with a=0.86779(8)and C=O．50923(5)nm．The isothermal section of this ternary system at 773 K consists of 23 single-phase regions，42 two-phase regions and 22 three-phase regions．The solid solutions this temaly system is formed by substitution of Ga for Fe．The maximum solid solubilities of Ga in ThFe2，ThFe3 and Th2Fe17 are 19.6，8.0，and 37.5 at％ respectively．The homogeneity ranges for the other compounds are small．