In order to investigate the dependency of gas-liquid two-phase flow on pipe scale, the transition characteristics of flow pattern and phase distribution were studied experimentally in upward air-water two-phase flow along a large vertical pipe (inner diameter D: 0.2 m, the ratio of pipe length to diameter L/D: 61.5). The experiments were conducted under the flow rate: 0.03 m/s less than or equal to superficial air velocity (at top of test section) less than or equal to 4.7 m/s, 0.06 m/s less than or equal to superficial water velocity J(L) less than or equal to 1.06 m/s. Flow pattern was observed and measurements were performed on axial differential pressure, phase distribution, bubble size and bubble and water velocities. The scale effect was discussed with small-scale data (D: 0.025-0.038 m). The flow conditions at which coalescence starts are almost the same as those found in small-scale pipes, but no large bubbles are observed in the region L/D < 20 which corresponds to the developing region of the axial differential pressure curves. The large coalescent bubbles were generated in L/D > 20. The churn flow is dominant in the large vertical pipe under the conditions where small-scale pipes have slug flow. The transition of phase distribution corresponds to the change of flow pattern. Large coalescent bubbles affect the phase distribution as similar to small-scale pipes but the following remarks are concluded as the scale effect: (1) under a low J(L) where small-scale pipes have a wall-peak phase distribution, a core-peak phase distribution is established, where some large eddies including bubble clusters fill up the pipe, (2) the large coalescent bubbles are developed along the test section via the churn bubbly flow where the phase distribution is a core peak one, whereas Taylor bubbles in small-scale pipes are generated at the vicinity of gas-liquid mixing region or are developed from the bubbly flow with a wall-peak phase distribution, (3) the wall-peak in the large vertical pipe is lower even under the same bubble size. The lower peak is considered to be related to the lower radial velocity gradient of water and the larger turbulent dispersion force. (C) 2000 Elsevier Science Ltd. All rights reserved.