An improved molecular thermodynamic model has been developed and applied to various pure, binary, and ternary mixed surfactants in aqueous solution containing salt (0-3 M). The effect of counterion activity and surfactant activity on surfactant aggregation is considered. The effect of counterion and co-ion specificity on aggregation properties is successfully evaluated. The predicted aggregation properties, including critical micelle concentration (cmc), micelle shape, micelle size, and sphere-to-rod transition, agree well with experimental data. The developed model provides a potential method to evaluate ion effects on aggregation properties of various surfactants in salt solution at various concentration levels, and thus will be of great value to guide the application of surfactants in industrial products and processes, such as corrosion prevention, oil recovery, and pharmaceuticals.