A two-dimensional, heat pipe transient analysis model, ’HPTAM’, is developed for simulating operations of fully-thawed heat pipes. The model is benchmarked using transient experimental data of a horizontal water heat pipe. The calculated steady-state water vapor and wall axial temperature profiles and the transient power throughput are in good agreement with measurements. Also presented and discussed are the calculated axial distributions of liquid and vapor pressures, effective radius of curvature of the liquid meniscus at the liquid-vapor interface, and liquid pooling and recession following step function heatup and cooldown transients of the water heat pipe.