Abstract: The radial distribution of air-water two-phase upward bubbly flow in a vertical circular tube (ID 50 mm) was investigated experimentally by using the optical fiber probe under ambient temperature and pressure. The results show that the radial distribution of void fraction changes with different superficial gas and liquid velocities. The void fraction is almost invariable along radius except in the near wall region where it has a peak and then decreases sharply at high superficial liquid velocity which is called “wall peak”. In the case of stagnant liquid, the increasing rate for void fraction becomes slower as the radial position varies from the center to the wall with the increase of superficial gas velocity. When the superficial gas velocity increases, the void fraction decreases radially from the core to the wall location, namely “core peak”. When the liquid velocity is low, the void fraction distribution behaves as the transition pattern. The relative deviration of the probe measured area-weighted averaged void fraction against that based on pressure drop is less than 10%.