Characteristics of co-current upward air-water two-phase flow patterns in vertical equilateral triangular channels with hydraulic diameters of 2.886, 1.443 and 0.866 mm were investigated experimentally. Flow regimes were identified by both visual observations using a high-speed motion analyzer and dynamic pressure-drop measurements. The experimental results show that the typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as dispersed bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters (d(h) = 2.886 and 1.443 mm). However, for the smallest channel (d(h) = 0.866 mm), dispersed bubbly how pattern, characterized by randomly dispersed bubbles in continuous liquid phase, was not found, although the other typical flow patterns remained in the channel. Moreover, the experiments reveal that, for the channel of ai, = 0.866 mm, a so-called capillary bubbly flow pattern, characterized by a single train of bubbles, essentially ellipsoidal in shape and spanning almost the entire cross-section of the channel, existed at low gas flow rates. It is Further found that in the slug flow regime, slug-bubbles were substantially elongated. Finally. flow regime transition boundaries for the triangular microchannels were compared with relevant flow regime transition models and correlations as well as the existing experimental data for small round tubes and square channels.