A physically based theoretical model of gas-liquid and steam-water two-phase flow in a horizontal tee-junction is developed. The model includes five independent equations: the mixture continuity equation, the vapor-phase continuity equation, the x direction momentum equation, the y direction momentum equation, and the energy equation. Any five unknown hydraulic parameters of the tee-junction can be solved in various ways, for virtually any choice of three well-posed boundary conditions. The uncertainty of the interfacial terms and the number of empirical constants that are used in the model are limited. The results of the calculations are compared with experimental data gathered from the literature. The comparison shows that the predictive ability of the model is reasonably good, except that the mass balance equation of the vapor phase is not suitable for some of the steam-water experimental data, and interfacial evaporation and condensation terms should be introduced into the model in the future.