Abstract: The vertical-downward two-phase flow has different interfacial structures and transport characteristics compared with vertical-upward two-phase flow. The air-water two-phase flow in vertical-downward pipes was investigated in the paper. The four-sensor conductivity probe was used to measure the distribution of local two-phase flow parameters (void fraction, interfacial area concentration, bubble diameter, bubble frequency, and bubble velocity) across pipe cross-sections at 7.5, 31.5 and 55.5 times pipe diameters. The experimental data were used to analyze the development characteristics of two-phase flow parameters across the pipe cross-section and along the flow direction, the effect of gas-phase superficial velocity, and the one-dimensional transport of the averaged two-phase flow parameters. The results show that the lift force points toward the pipe center, which results in a center-peaked void fraction distribution in vertical-downward two-phase flow. The kurtosis of the void fraction profile and the interfacial area concentration profile increase with the gas-phase superficial velocity. The vertical-downward two-phase flow becomes nearly fully developed at 31.5 times pipe diameters.