Utilizing the converging-diverging geometry and Bernoulli principle in general, a Venturi tube type bubble generator that produce small bubbles due to the characteristics flow phenomenon in the nozzle is able to generate a high number density of less than 100 µm microbubbles. The bubble diameter distribution is influenced by the water flow rate, gas flow rate supplied to the tube, and tube geometry. Studies show that the water flow rate gives smaller size bubbles as it increased, while the increase of gas flow rate gives the opposite result. However, in most research done for geometry effects on bubble diameter distribution, the cavitation effects to the bubble diameter distribution in a bubbly flow is not discussed. Therefore, the aim of the present study is to observe cavitation presence in the Venturi tube and its effect on bubble breakup characteristics. In this experiment we use two different throat sizes of Venturi tube with gas injected upstream. Gas flow ratio of 0~0.016 and liquid superficial speed from 1.1~3.0 m/s is chosen as the experiment condition. Experiment shows cavitation occurs in both tubes at different superficial velocity rate in single-phase flow. Under bubbly flow condition, supersonic flow, shows cavitation number less than 1 with higher pressure loss coefficient and formed higher number of smaller bubbles in a Venturi tube.