An evaporator model has been developed for investigating heat and mass transfer in a flat evaporator of a copper-water loop heat pipe. It takes into account heat-transfer processes in the active zone, the barrier layer of the wick, the wall and the compensation chamber. The problem was solved by the finite difference method with the use of a nonuniform grid adapted to the configuration of a flat evaporator and its geometric peculiarities. The model examined a uniform and concentrated heat supply to the evaporator. In the first case the heat-supply area was 9 cm2, in the second 1 cm2. Numerical simulation of temperature fields of the evaporator has been performed for a heat load range from 20 to 900 W. Data have shown that a decrease in the heating area at a fixed heat load results in increasing temperature on the evaporator wall under the heater. With a concentrated heat supply the evaporation zone spreads beyond the “heating spot”, and at heat loads above 400 W the drying of the part of the wick under the heater begins. No disturbance of serviceability of the heat-transfer device is observed in this case as the crisis of operating heat transfer is of a local nature. In such conditions the evaporation front shifts in the direction of peripheral areas located beyond the “heating spot”. Under uniform heating the wick drying was not registered in the investigated heat load range.