The physical operational mechanisms for the evaporator of capillary pumped loops are described, and possible working limits are identified. A generalized mathematical formulation for the flat-plate type evaporator is given, which includes liquid flow and heat transfer in the porous wick structure, three-dimensional vapor flow in the groove, and heat transfer in the cover plate. The liquid and vapor flows are coupled via the interfacial mass flux, and the entire evaporator is solved as a conjugate problem. Numerical results under different working conditions and with different fluids are presented, which provide guidance for the evaporator design of capillary pumped loops.