Loop Heat Pipes (LHPs) have been base-lined for thermal management in the next generation of large communication satellites due to their ability to effectively transport energy using two-phase heat transfer. To incorporate LHPs in thermal design, a subroutine was developed to work with a nodal thermal analyzer for transient predictions of LHP operating temperatures. The thermal analyzer (e.g., SINDA or IDEAS/TMG) is used to advance the thermal solution in time, while the subroutine provides the time-varying fluid boundary conditions and adjusts thermal couplings by solving the conservation equations of mass, momentum and energy.The subroutine was exercised in conjunction with a nodal thermal analyzer to predict the fluid temperatures in a loop heat pipe during transient changes in power and sink temperature. A laboratory test was performed with an ammonia LHP to collect data for verifying the model. Presented in this paper is a description of the model and the test data used to examine the model’s prediction capability.