A thermodynamic model of flat heat pipe is developed based on the laws of thermodynamics. Major reasons for entropy generation, which is considered as a significant parameter on heat pipe performance, are temperature difference between hot and cold reservoirs, frictional losses in the flow of working fluid, and vapor temperature and pressure drop along heat pipe. The objective of the present work is to minimize the entropy generation in a flat heat pipe. The physical situation is formulated as a non-linear programming problem with non-linear objective function and constraints. Using available software, the optimum values of selected design variables are arrived. The effect of heat load, adiabatic length, etc. on the optimum design variables and corresponding entropy generation are studied. (C) 2010 Elsevier Ltd. All rights reserved.