A three dimensional modeling of a loop heat pipe (LHP) evaporator is presented. The model is developed to take into account different geometrical configurations of the evaporator, as well as the non-symmetrical temperature and heat flux distributions in a cylindrical LHP evaporator which is usually neglected. Both Convection and conduction phenomena are considered in the porous wick. The evaporation heat transfer coefficient is calculated at the vaporization interface. This 3D modeling approach is integrated into a global model of the whole loop heat pipe and compared with experimental measurements. Various modifications of the evaporator design have been considered: the most important improvement of the performance is obtained by optimizing the disposition of the axial vapor grooves as well as the saddle, or radial vapor grooves.