A stable numerical procedure is developed to analyze the transient performance of flat heat pipes for large input heat fluxes and high wick conductivity. Computation of flow and heat transfer in a heat pipe is complicated by the strong coupling among the velocity, pressure and temperature fields with phase change at the interface between the vapor and wick. A structured collocated finite volume scheme is used in conjunction with the SIMPLE algorithm to solve the continuity, energy and momentum equations. In addition, system pressurization is computed using overall mass balance. The stability of the standard sequential procedure is improved by accounting for the coupling between the evaporator/condenser mass flow rate and the interface temperature and pressure as well as the system pressure. The improved numerical scheme is applied to a flat two-dimensional heat pipe and shown to perform well. Parametric studies are performed by varying the vapor core thickness of the heat pipe and the heat input at the evaporator. The model predictions are validated by comparing the heat pipe wall temperatures against experimental values.