Using a high-speed digital microscope, visualization experiments of the vapor-liquid two-phase flow and heat transfer in an axially grooved heat pipe are conducted to investigate the evaporation regimes that occur at small tilt angles. The effects of the heat load and tilt angle on these evaporation regimes are analyzed. The evaporation regimes are further quantitatively characterized with a regime diagram developed based on the Bond number and Weber number. The results indicate that, unlike in a horizontal position, the evaporation regime of a grooved heat pipe with a small tilt angle can include corner-film evaporation induced by the combination of gravity and capillary force in addition to pool-surface evaporation and fin film evaporation. Corner-film evaporation is the characteristic evaporation regime of an axially grooved heat pipe-with-a-small tilt-angle, and it exhibits-random-oscillatory motion of the vapor-liquid interface. Pool-surface evaporation is observed at small Weber numbers, corner-film evaporation occurs at large Weber numbers when the Bond number is approximately greater than 0.003, and fin-film evaporation occurs at medium Weber numbers. (C) 2018 Elsevier Ltd. All rights reserved.