The iso-viscosity curves of liquid Fe.Ni.Co ternary alloys at 1600 8C were investigated with considering the excess viscosity of ternary alloy by the means of three models (Kohler, Toop and Chou) from the measured sub-binary data in this work. Excess viscosities were used instead of excess thermodynamic properties in geometrical models, and the excess viscosities of the sub-binary systems were fit by using the 3rd degree (n = 3) Redlich.Kister polynomial. The increase of Ni content results in the decrease of the viscosity of the ternary alloy, while Fe has opposite effect. When the molar content of Ni within the alloy does not exceed 50%, the increase of Co lowers the viscosity. Once its content is above 50%, Co can promote the increase of the viscosity of the ternary alloy. Similarity coefficients of the Fe.Ni, Fe.Co and Ni.Co three binary systems mentioned in the Chou model have been calculated and their values are 0.82, 0.20 and 0.47, respectively, showing that the Fe.Ni.Co ternary system is not exactly the ‘‘Kohler model’’ or ‘‘Toop model’’, so both Kohler model and Toop model cannot obtain the accurate predicted values. The predicted iso-viscosity curves calculated by the Chou model should be recommended. Some activation energies of the ternary alloy were calculated by the three models and compared with three sub-binary systems, and the evaluated activation energies calculated by the Chou model should be recommended. Except the sample AE3 and AE6, other activation energy values predicted by three different models are similar by a narrow margin. The activation energies for AE3 and AE6 are the lowest and highest, respectively, because of the mix effect of the ternary alloy.