The lesson learned from the severe accident of Fukushima Daiichi nuclear power plant shows that the residual heat generated from nuclear spent fuel should be cooled properly. In order to absorb that residual heat when station blackout occurs, the wickless-heat pipe is proposed to be used as an alternative to the passive cooling system in nuclear spent fuel pool. The objective of this research is to simulation the effect of initial pressure and evaporator filling ratio as factors that influence the thermal performance of wickless-heat pipe. The simulation results will be validated with experiment results. The wickless-heat pipe model was built and simulated using nuclear thermal-hydraulic code RELAP5/MOD3. 2. The wickless-heat pipe model is built similarly, and it has same geometry with experiment test section. In the simulation, the initial pressure inside the wickless-heat pipe and evaporator filling ratio are varied. The initial pressure is varied on-54 cm Hg,-64 cm Hg, and-74 cm Hg, and filling ratio of the evaporator is varied on 40%, 60%, and 80%. The heat load of the evaporator, coolant temperature, and coolant volumetric flow rate were kept constant. The results obtained show that thermal resistance of wickless-heat pipe simulation model is 0.005 C/W. It is showed that simulation model results have good agreement with experiment results, and it can be used to simulate wickless-heat pipe heat transfer phenomena with different values of the input parameter. The RELAP5/MOD3. 2 simulation model has been verified by the experimental result of a steady state condition.