The paper investigates the transient processes of heat and mass transfer in a cylindrical evaporator of a loop heat pipe (LHP) during the device start-Lip. One of the most "arduous" prestart situations, which is characterized by the absence of a liquid ill the evaporator central core and filled vapor removal channels, has been considered. With such liquid distribution a Successful start-up of an LHP becomes possible only after formation of the vapor phase in the vapor removal channels and their liberation from the liquid. The aim of the investigations is to determine conditions that ensure the boiling-Up of a working fluid in vapor removal channels. The problem was solved by a numerical method. Simulation of start-up regimes has been performed for different heat loads and different structural materials of the evaporator. Copper, titanium and nickel wick have been examined. Calculations have been made for three different working fluids; water, ammonia and acetone. Account has been taken of the conditions of heat exchange between the compensation chamber and surrounding medium. (c) 2008 Elsevier Ltd. All rights reserved.