The transient performance of a flat heat pipe (FHP) used to cool multiple electronics components, is presented in this paper. The fluid flows in both wick and vapor core were computed using a transient 2D hydrodynamic model (T2DHM). This model was coupled with a transient 3D thermal model (T3DTM) of the FHP wall, designed to calculate the heat transfer through the wall. An interesting procedure for solving the governing equations for the heat and mass transfers inside the FHP is proposed. The phase change mechanisms at the liquid-vapor interface are included in this procedure through the Clausius-Clapeyron law. During a start-up, the T2DHM is able to predict the velocity and pressure distribution of the liquid and the vapor, and thus the transient response of the FHP. (C) 2008 Elsevier Ltd. All rights reserved.