The heat and mass transfer in a loop heat pipe evaporator with microgrooves are investigated in a three-dimensional numerical model. The design of the microgroove wick must consider the pressure loss of the vapor. The simulation obtains the detailed pressure distribution of the vapor flow in the grooves. The simulated heat flux distribution in the evaporator revealed that the applied heat flux concentrates along the three-phase contact line (TPCL) within the casing, wick, and grooves. The effect of TPCL length was investigated in three microgroove wicks with circumferential and axial grooves and a classical wick with only axial grooves. The heat-transfer coefficient initially increased with TPCL length increasing, but thereafter decreased because when the TPCL becomes too long, a large pressure loss occurs in the grooves. The developed model was validated experimentally. In both the model and experiment, the heat-transfer coefficient was locally maximized at a certain TPCL length. The proposed method is expected to provide a simple approach for wick design; especially, the wick shape can be optimized merely by varying the TPCL length. The effect of thermal conductivity of the wick material is also discussed.