In order to determine the heat transfer performance and the mechanisms of separate heat pipe (SHP) under various operating conditions covering two-phase states and transcritical states, experimental investigation was carried out in this study. The effect of charging mass, cold bath temperature, heat load and height between evaporator and condenser on heat pipe was analyzed and the thermal resistance was calculated to characterize the heat transfer performance. Results showed that the increase of head load, the decrease of height difference and the increase of condenser temperature led to increases in thermal resistance. Appropriate charging mass was seen to be the most important factor in terms of optimum heat transfer performance, whereas too much or too little charging mass led to the subcooling or superheating in the evaporator, which was found to have a direct impact on heat transfer performance. (C) 2018 Elsevier Ltd. All rights reserved.