Two-phase closed thermosyphons are widely-used in various energy intensive industries including chemical, transportation and other industries for their inherently high heat transfer efficiency, simple construction and reliable operational performance. However, the performance parameters of two-phase closed thermosyphons including the distribution of internal pressure, steam and liquid phase mass fractions, velocity and wall temperatures are obtained primarily via experimental investigations. In this paper numerical methods are discussed and a two-fluid model is employed to describe the two-phase flow and heat transfer processes in a two-phase closed thermosyphon. The IPSA (Inter Phase Slip Algorithm) algorithm is employed to solve the coupled interactions of steam and liquid phases along the phase interface. Flow patterns and distribution of parameters under different conditions are predicted with numerical results agreeing well for the most part with experimental results. Thus, the numerical method and solution procedures are claimed to be of practical utility and can in essence be used profitably to simulate flow and heat transfer phenomena in thermosyphons and other types of heat pipes.