In permafrost regions, two-phase closed thermosyphons (TPCTs) can be used to release heat from permafrost. TPCTs are thus embedded in foundations of infrastructures for preventing the permafrost thawing. The installation of a TPCT, however, greatly influences the efficiency of releasing heat, which has not been fully understood. To find an optimum installation for TPCTs, we experimentally studied the temperature differences and thermal resistances of a TPCT that was sequentially installed at an inclination angle of 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90 degrees at subzero temperatures. It is found that the inner startup temperature difference varied irregularly with inclination angle. It is also found that at evaporator and condenser sections, the thermal resistance of the wall contributes negligibly to the thermal resistance of the corresponding section. Furthermore, the thermal resistances vary with inclination angle in the evaporator, adiabatic, and condenser sections. The adiabatic has a thermal resistance that is comparable to the thermal resistance of the evaporator and condenser sections except the inclination angle of 90 degrees. The TPCT has the minimum total thermal resistance when the inclination angle is about 20 degrees. The results could be helpful for the optimal application of TPCTs in permafrost engineering. (C) 2021 Elsevier Ltd. All rights reserved.