Activity of tin in liquid Sn-Bi-Sb was derived by EMF measurement of galvanic cell with fused salts electrolyte in the temperature range of 700 to 1000 K in the whole composition range. Activity of tin at 900 K shows very small positive deviation from Raoult's law for Sn-Bi alloys and moderately negative deviation for Sn-Sb alloys. Activity of ternary alloys along Sb_yBi_(1－y)-Sn (y＝0.25, 0.50 and 0.75) shows negative deviation and the deviation becomes large with increasing Sb content. Excess free energy of mixing is derived using Darken's method for Gibbs-Duhem equation and the values are compared with those by model calculations based on the three constitutive binary data.