Abstract An experimental study was performed on the local structure of atmospheric upward-moving air–water two-phase flow in a vertical large-diameter square duct with a cross-section of 100 mm × 100 mm and a height of 3 m by using a four-sensor probe at the axial position of z/DH = 29.2. The flow regime was simultaneously observed by a high speed camera at z/DH = 12.5. The four-sensor probe method classifying spherical and non-spherical bubbles was applied as a key measurement way to obtain local parameters such as 3-D bubble velocity vector, bubble diameter and interfacial area concentration, which are necessary for people to know the internal flow structures as well as local void fraction change. The local measurements were performed at 66 points within an octant symmetric triangular area of the cross-section, which agreed well with area-averaged void fractions from differential pressure gauges and the superficial gas velocity from air flow meters. Both the local void fraction and interfacial area concentration indicated radial core-peak and wall-peak distributions at low and high liquid flow rates respectively. Main flow component of the bubble velocity vector showed a power-law distribution shape with its center peak increasing with the liquid flow rate. The 2 components of the bubble velocity vector in the cross-section revealed that there exists a rotating secondary flow in the octant symmetric triangular area and the magnitude of the rotating secondary flow increases with the liquid flow rate. Some of constitutive correlations of drift-flux model for rectangular flow channels and large-diameter circular flow channels and interfacial area concentration correlations for small-diameter and large-diameter circular flow channels are selected and reviewed to study their predictabilities against the present experimental data. The comparison study shows that the drift-flux correlations by Hibiki and Ishii (2003) reasonably predict void fraction and the interfacial-area-concentration correlations by Hibiki and Ishii (2002) and Shen and Hibiki (2015) can be recommended for the prediction of interfacial area concentration in the bubbly flows in large-diameter square ducts.