Abstract: Heat pipe cooled reactors (HPRs) have been considered as one of the most promising candidates for deep space and deep-sea missions due to their advantages of simple structure, high power density and high reliability, etc. To investigate the transient characteristics of such heat pipe cooled reactors, including startup, shutdown, power transients and accident conditions, it is necessary to develop suitable and efficient models for describing the core, the heat pipe and the power conversion system. Especially, for the startup process, an accurate and efficient model for the simulation of high-temperature heat pipe startup from the frozen state is indispensable. In this study, two transient models based on the dusty gas model (DGM) were developed. The first model (model 1) solved the mass and momentum equations of vapor flow, while the second model (model 2) simplified the vapor flow as a 1D steady-state heat conduction problem using an equivalent network model. The models considered the evaporation/condensation flux at the vapor/liquid interface using the kinetic theory of gases. The wick and wall were modeled using an improved network model, which took into account the phase transition of the working fluid in the wick. Different methods were used to solve these models in this paper. For the model considering the vapor flow, the finite-difference discretization scheme and the SIMPLEC algorithm were used to solve the governing equations. For the equivalent network model, a loosely coupled numerical method is employed. The solution of wick and wall equations was in a transient state, while the equivalent heat conduction equation of the vapor flow was solved in a steady state mode. The alternating direction implicit (ADI) was adopted to solve the equations for the wick and wall regions. The startup experiments of high-temperature heat pipes with different working fluids are simulated to validate the accuracy of these models. The results indicate that the simulation results agree well with the experimental data. Compared with the flat-front startup model, the temperature distribution calculated by model 2 is more accurate, and the description of startup is more plausible. Meanwhile, model 2 gives quite reasonable results although it is less accurate than the model 1. The calculation efficiency of the model 2 is significantly improved compared to model 1. Consequently, in the feasibility study stage of an HPR system, the simplified equivalent network model (model 2) with considering both accuracy and efficiency is suitable for the simulation of heat pipe startup.