Abstract: The formation mechanism of radial void fraction distribution of gas-liquid two phase flow in a vertical circular tube was investigated experimentally by using an optical fiber probe under ambient temperature and pressure. Experiments were conducted in a tube with inner diameter of 100 mm, and with the gas and liquid superficial velocity covering the ranges of 0.0042-0.053 m·s-1 and 0.071-0.213 m·s-1, respectively. The results show that local void fraction shows a ‘‘core’’ or a "wall" peak distribution along the radius with different gas and liquid flow rates. The analysis of the wall force and lift force acting on a bubble shows that the two forces are of great importance in leading to a "core" or "wall" peak distribution of the void fraction due to their key role in determining the transverse movement of a bubble.