This paper considers liquid film thickness and heat transfer in two-phase closed thermosyphons. A new mathematical model is presented for the better understand and design of various types of two-phase closed thermosyphon. Thermosiphons are used in many industrial applications such solar collectors, gas turbine blade cooling, water and soil freezing and in HVAC systems related to the energy development. The steady-state response of the thermosyphon to various saturation temperatures is discussed. The mathematical modeling of the fluid flow and heat transfer in the vapor space and liquid film region are described. The momentum equations of both phases coupled and highly non-linear are derived and solved numerically to calculate liquid film thickness and velocity. The liquid film thickness gets bigger when the saturation temperature decreases. In addition, the results show that the pressure gradient changes rapidly when the saturation temperature decreases and more vapor is produced at lower saturation temperature.