Abstract: Based on energy balance, pressure balance, mass conservation and geometric constraint, the steady state model of loop heat pipes was established and solved. The modeling results not only confirms the steady state operating characteristics of loop heat pipes introduced in many literatures, but also brings forward viewpoint to the problems below:the analysis of the heat leak of the evaporator, the reason of the generation of the variable heat conductance zone and the constant heat conductance zone, the variation of the heat conductance in the constant heat conductance zone and the calculation of the mass of the working fluid charged and the volume of the compensation chamber. Results show that the heat leak of the evaporator for heating the liquid through the wick can not be neglected at high heat load;adverse gravity elevation and the heating from environment to the return liquid are the reason of the generation of the variable heat conductance zone;the heat conductance in the constant heat conductance zone decreases as the heat load increases at high heat load, and can reach the critical heat load, which can be avoided by the match of the mass of the working fluid charged and the volume of the compensation chamber.