Abstract: The constant electric conductivity curves of solid solutions in the SrTiO₃-La_0.3Sr_0.7TiO₃-(La_0.3Sr_0.7)_0.93TiO₃ ternary system were obtained by estimating the electric conductivity of the solid solutions by a mass triangle model. Some typical compositions of the solid solutions were selected to verify the calculated results. The experimental values are consistent well with the predicted ones, demonstrating the feasibility of a mass triangle model in predicting the electric conductivity of mixed conductor.