The gravity-capillary evaporation phase change in microgrooves inside a heat pipe is experimentally investigated by a visualization system. The evaporation regime, startup temperature response, and evaporation thermal resistance for gravity-capillary evaporation are evaluated and compared with those for capillary evaporation. The results identify oscillatory motion of vapor-liquid interface for gravity-capillary evaporation in rectangular microgrooves arising from the competition between gravity, the capillary force, and the shear force. In addition, the presence of gravity introduces the transition from fin-film evaporation regime to corner-film evaporation regime in rectangular microgrooves, and hence results in an overshoot startup and temperature oscillation during the process of gravity-capillary evaporation. Interestingly, the corner-film evaporation improves the evaporation phase change performance in the microgrooves. (c) 2018 American Institute of Chemical Engineers