The flow structure of gas-liquid two-phase flow in vertical annulus channel has been investigated. The inner and outer diameters of the annular channel were 19.1 and 38.1 mm, respectively. The total height of the test section was 4.37 m. Nineteen inlet flow conditions were selected, which cover bubbly, cap-slug, and churn-turbulent flows. The local flow parameters, such as void fraction, interfacial area concentration (IAC) and bubble interface velocity, were measured at nine radial positions within the gap of the annulus at z/Dh = 230 of the test section. The flow regimes of the flow conditions, which were based on visual observations, were compared with several flow regime maps. In addition, the local measurements were used to calculate distribution parameter, C-0 in drift-flux model, and area-averaged IAC. A new correlation of Co was proposed based on the experimentally obtained C-0 values. This correlation was tested in the drift-flux model successfully along with Ishii's drift velocity correlations. The area-averaged IAC values were compared with the most widely used models. The advantages and drawbacks of these models were highlighted. (c) 2008 Elsevier Ltd. All rights reserved.