Experiments were carried out to find the effects of dissolved gas pressure, liquid flow rate and nozzle geometry on the bubble generation when saturated water was depressurized through a nozzle. A new method, high speed camera system was developed to measure the generated microbubbles dynamically. On the basis of the laws of ideal gas and solution, theoretical generated gas flow rate was deduced, while the Smoluchowski＇s equation was applied to describe the kinetics of bubble nucleation. It was found that the size distribution of nucleated bubbles was of skewed distribution. An explanation to this phenomenon was made and the Gamma function distribution was employed for mathematical simulation. The results show good agreement between the experimental data and the predictions by proposed model.