The heat and mass transfer in the capillary porous structure of a loop heat pipe (LHP) is numerically studied and the LHP boiling limit is investigated. The mass, momentum and energy equations are solved numerically using the finite element method for an evaporator cross section. When a separate vapor region is formed inside the capillary structure, the shape of the free boundary is calculated by satisfying the mass and energy balance conditions at the interface. The superheat limits in the capillary structure are estimated by using the cluster nucleation theory. An explanation is provided for the robustness of LHPs to the boiling limit. (c) 2006 Elsevier Ltd. All rights reserved.