•An ammonia LHP with flat evaporator and long heat transfer distance was designed.•The loop could operate stably in the range of heat loads from 2.5 W to 180 W.•The evaporator inlet temperature suffered a staged decline due to vapor expansion.•Two different processes related to vapor phase and heat leak were observed.•A minimum LHP thermal resistance was 0.252 °C/W at heat sink temperature of 10 °C. Loop heat pipes are passive heat transfer devices which can meet the heat dissipation requirement of high-power electronic devices in aerospace and terrestrial applications. This paper investigates the operating characteristic of a stainless steel-ammonia loop heat pipe with a flat disk evaporator. A biporous wick made from sintered nickel powders was used to produce the capillary force. The heat transfer distance was 1.6 m and the allowable heater surface temperature was below 70 °C. Tests demonstrated that the loop could operate under a heat load ranging from 2.5 W to 180 W (heat flux 0.15–10.8 W/cm2) at heat sink temperature of −10 °C. In addition, variable conductance mode and constant conductance mode existed, and no obvious temperature overshoot or pulsation was observed. The evaporator inlet temperature went through a staged decline due to the effect of initial driving force of vapor expansion during the start-up process. Meanwhile, under the synergy of heat leaks from heater surface and long transport line, whether the vapor phase would form inside the compensation chamber could result in a large discrepancy in temperature trends. The minimum evaporator thermal resistance was 0.096 °C/W at heat sink temperature of −10 °C and the minimum LHP thermal resistance was 0.252 °C/W at heat sink temperature of 10 °C.