This paper considers pressure gradient and heat transfer in two-phase heat pipes. A new mathematical model is presented for the better design and manufacturing of various types of two-phase heat pipes and relevant to the many applications in HVAC related to energy development. The steady-state response of the heat pipe to various heat fluxes in the evaporator and condenser sections are discussed. The mathematical modeling of the fluid flow and heat transfer in the vapor space and liquid film region are described. The continuity and momentum equations of both phases coupled and non-linear are derived and solved to calculate flow variables such as mass flow rate of liqid film, pressure gradient as well as wall shear stress.