We investigated slug dynamics in nucleate pool boiling on downward-facing surfaces using high-speed camera visualization. The behavior of a single slug was observed according to orientation angle (120, 135,150,160,170 degrees) and heat flux conditions, to hear critical heat flux. Using image processing, the translation speed, frequency, departing diameter of slug, and vapor generation rate were determined. In nucleate boiling on a downward-facing surface, the slug deformed in the normal direction to the surface and slowed down with the tangential direction of the surface, compared with that on an upward-facing surface (0 degrees). Accordingly, a thinner and wider slug departed from the heating surface at a lower speed, as the orientation angle increased. Thus, the measured translation speed of the slug was strongly dependent on the orientation and radius of the slug. Moreover, there was a strong relationship between the departing frequency and diameter of the slug and the vapor generation rate from the heating surface. Based on the measured vapor generation rate from images, heat flux by evaporation was predicted, resulting in slight overestimation of the applied heat flux. These findings indicate that evaporation represented most of the heat transfer in downward nucleate boiling, rather than quenching. . (C) 2017 Elsevier Ltd. All rights reserved.