•Cylindrical evaporator-CC coupling structure was visually analyzed at three tilt angles.•The performances of the vapour-liquid interface in the coupling structure was studied.•The bubble generation and its movement in the coupling structure were observed and studied.•Temperature fluctuations of the LHP were related to the oscillations of vapour-liquid interface.•The conclusions are helpful for the heat transfer mechanism of the LHP in gravity applications. To visually analyze the flow and heat transfer characteristics of the working fluid in a loop heat pipe (LHP), the evaporator-compensation chamber (CC) was designed as a half-sectioned cylindrical structure sealed with a glass window. Using acetone as the working fluid, visualization experiments were performed under different heat loads and tilt angles. The vapour-liquid distribution in the evaporator-CC coupling structure and the temperature variation characteristics of the LHP system were investigated at three tilt angles of the evaporator-CC coupling structure. According to the experimental results including the startup and steady-state operation, some important conclusions have been drawn as summarized below: (1) with the subcooled liquid outflow from the bayonet tube, the vapour-liquid interface in the evaporator-CC coupling structure rises when the heat load is applied to the evaporator; (2) the bubble generation inside the porous wick becomes more intense with an increased heat load, and there are various bubble movement patterns at different tilt angles; (3) the temperature fluctuation of the LHP system is closely related to the oscillation of the vapour-liquid interface in the evaporator core.