Abstract: Modified photo-electric capillary probe technique with improved sampling accuracy was used to measure bubble size distribution in an aerated stirred vessel. Simulated calculation indicates that the ratio of Sauter mean diameter or standard deviation to maximum diameter can approximately keep constant despite of change in operating conditions. The main factors governing the bubble size in impeller region and in circulation region have been deduced according to turbulence theory. Experimental findings show that the probability density of bubble diameter approaches that of normal distribution in air-water systems. There exists obvious difference in magnitude between bubble diameter in impeller region and that in circulation region. The quantitative relations between the Sauter mean dimeter in different regions and the power per unit volume, superficial gas velocity and the ratio ot impeller diameter to tank diameter have been given.