In view of the practical importance of the application of two-phase flow in a large diameter pipe to various fields of engineering, the characteristics and phase distribution patterns of two-phase flow in a vertical large diameter pipe have been experimentally and theoretically studied for various flow conditions. The local measurements for the interfacial parameters (void fraction, Sauter mean diameter and pressure loss) in a vertical upward air-water two-phase flow in a pipe with 0.2 m in inner diameter and 24 m in height have been performed by using the optical probes and differential pressure transducers. The two-phase flow characteristics have been analyzed with experimental data, which shows that the phase distribution patterns in the vertical large diameter pipe can be divided into two basic patterns, namely, wall peak and core peak. With the application of the concept of skewness, the two-phase distribution patterns have been quantitatively distinguished by establishing a phase distribution pattern transition criterion. An empirical relation for the phase distribution transition from wall peak to core peak was fitted by using the phase distribution pattern transition criterion and the present experimental data and verified by other researchers' experimental data. This study also showed that there existed the flow plugging phenomena in the low region of the test section at high superficial gas velocity conditions in the vertical large diameter pipe. (C) 2004 Elsevier Ltd. All rights reserved.