The use of the two-phase closed thermosyphons (TPCTs) is increasing for many heat transfer applications. In this study, the development of a numerical model is presented with the purpose of determining the transient characteristics of a TPCT. The model includes the heat transfer through the wall, vapour core, liquid pool and the falling condensate film. The complete two-dimensional conservation equations for mass, momentum, and energy are solved using finite volume scheme for the vapour flow and pipe wall. The liquid film is modelled using a one dimensional quasi-steady Nusselt type solution. The model is validated by comparison with existing experimental data. The effect of filling ratio (volume of working fluid to evaporator section) for a copper thermosyphon has been investigated. Water is used as a working fluid for different filling ratio from 20 to 100%. The investigations are aimed to determine working thermal performance of a TPCT at different filling ratio for solar energy applications.