In order to address the bubble formation and movement in air-water two-phase flow, single bubble rising in stagnant water is experimentally studied by digital image processing. Bubbles are released individually from the submerged orifices with different diameters (1.81 mm, 2.07 mm, 2.98 mm, 3.92 mm) at different detachment frequency. Images are recorded by a high-speed video camera and processed by digital image processing technique. The factors impacting the formed volume of bubble are discussed. The experimental results showed that a threshold of gas flow rate (400 mm(3).s(-1)) divides the bubble formation into two regimes: the constant volume regime and the growing volume regime. Especially for the growing volume regime, the surface tension is taken into account. The bubble volume is consisted of two parts: the surface tension impacting part and the gas volume flow rate impacting part. An improved correlation for bubble volume prediction is developed for the two regimes and better coincidence with the experiment data than the previous models is obtained.