This paper developed a new mathematical model to investigate the heat transfer as well as wick's thickness of a heat pipe. The model was established by conservative equations of continuity, momentum, and energy in the thermal boundary layer. Using a similarity variable, the governing equations were changed to a set of ordinary differential equations and were solved numerically by the forth-order Runge-Kutta method. The flow variables, such as velocity components, wick's thickness, and Nusselt number, were obtained. The results show that the Nusselt number is proportional to the square root of the Darcy-modified Rayleigh number and to the distance from the edge of the condenser surface. Furthermore, the thickness of the wick material depends on the Jakob number and is proportional to the heat transfer between the wall and liquid film. © 2018 Sharif University of Technology. All rights reserved.