The optimal working fluid inventory of a prototype of a cryogenic loop heat pipe (CLHP) is studied in this paper. The CLHP prototype operates within a temperature range of 77-126 K with nitrogen as the working fluid. The possible fluid inventory range is calculated based on analysis of phase distribution of vapor and liquid within the loop in a normal operation mode. Furthermore, a series of experiments are carried out with various filling pressures of the gaseous nitrogen at room temperature for validation of the fluid inventory range and identification of the optimal fluid inventory. The experimental results are compared with the calculation results, and the effect of different filling pressure on the performance of the CLHP is analyzed. (C) 2013 Elsevier Ltd. All rights reserved.