Loop heat pipes (LHPs) are efficient two-phase heat transfer devices that have found many space and terrestrial applications. This work addresses our insufficient understanding of LHP operation under gravity-assisted attitude, i.e. the condenser is located higher than the evaporator. A steady-state mathematical model of a LHP under gravity-assisted operation was established based on two driving modes: gravity driven mode and capillarity-gravity co-driven mode, determined by a defined transition heat load. The model was validated by the experimental results, and was employed to predict the operating characteristics of a LHP under the gravity-assisted attitude. Comparing to LHPs operating under horizontal or antigravity attitudes, some distinctive features have been identified, which include: i) the total mass flowrate in the loop shows a unique V-shape with the increase of applied heat load; ii) the steady-state operating temperature is much lower under the gravity driven mode, and is in similar values under capillarity-gravity co-driven mode and iii) the thermal conductance of the LHP increases with increasing positive elevation especially in the variable conductance zone. Such results contribute greatly to the understanding of the complicated operating principle and characteristics of LHPs especially for terrestrial applications.