Abstract: The experimental study was performed on characteristics of interfacial parameters radial distribution of air-water bubbly flow by using a two-head optical fiber probe in a vertical circular tube with the inner diameter of 50 mm. The gas and liquid superficial velocities cover the ranges of 0.004-0.05 m/s and 0.071-0.283 m/s, respectively. The results show that the local interfacial area concentration (IAC), bubble frequency and void fraction nearly have the same distribution, that is, the three interfacial parameters almost keep constant along radius except in the near wall region where it falls to a low value with a low gas velocity; as the gas velocity increasing, the interfacial parameter has a peak value in the core region of circular tube. In the experiment, few bubble coalescence and breaking up occur, and the bubble Sauter mean diameter distribution in the radial direction is approximately uniform and changes little with gas and liquid velocities. The lateral bubble force in cross section is considered to explain the local parameters distribution pattern.