Results of model calculations for evaporator of loop heat pipe in the field of gravity are presented. Two-dimensional partial differential equation problem in cylindrical geometry was solved in these calculations. The main problems to solve were to verify and refine existing analytical models of processes in evaporator and to estimate influence of natural convection on the heat and mass transfer in evaporator. Heat pipe with vertical orientation of cylindrical evaporator above condenser is taken into consideration. Water, acetone and ammonia are used as heat agents. The system of partial differential equations was solved using Peaceman - Rachford scheme (ADI - method). In order to estimate influence of natural convection on heat-mass transfer processes calculations with and without natural convection in compensation cavity were made. Influence of input power level, thickness of capillary structure, length of liquid channel embedded in compensation cavity is analyzed. The main results of the LHP modeling are: natural convection essentially increases temperatures of the free surface in the compensation cavity; strong mixing in water LHP takes place for all usable values of input power; suppression of natural convection can be recommended.