This study provides a visual observation of the two-phase flow patterns of the air-water mixtures inside two 3.0 mm and two 6.9 mm smooth tubes subject to the influence of vertical return bend. The effects of tube size (D = 6.9 and 3 mm) and curvature ratio (2R/D = 3 and 7) are examined. The total mass flux is from 50 to 800 kg/m(2)s. Some unique phenomena such as flow reversal, freezing slug are observed for the vertical return bend having D = 6.9 mm that are absent in previously reported results for horizontal configurations. However, the flow reversal and freezing slug phenomena are hardly seen at a smaller diameter tube (D = 3 mm) due to the influence of the flow inertia and surface tension. For D = 6.9 mm, the elongated bubble across the return bend reveals a separate and re-merged phenomenon but this phenomenon is nearly gone for D = 3 mm. Unlike that of D = 6.9 mm, for the same mass flux and vapor quality with D = 3.0 mm, there is no great difference of the two-phase flow patterns between the flow entering in the upper tube or the flow entering from the lower tube. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.