Venturi channels taken as bubble generators own merits of simplicity in structure, high efficiency, and high reliability. A visualized investigation was carried out on bubble transportation and breakup in two small rectangular Venturi channels with the throat sizes of 1 mm × 1 mm and 1 mm × 2 mm, respectively. Experiments were conducted under ambient conditions with air and water as the working fluids. The experimental results indicate that bubble transportation and breakup in the Venturi channel with the throat size of 1 mm × 1 mm presents some different features compared with the other one: under the same average liquid velocity in the throat, bubbles own higher initial velocity than the average liquid velocity before entering the diverging section, and remain this trend till they are split; a binary breakup occurs to the bubbles prior to their final collapse in the recirculation region due to the jet flow in the backward of the bubbles. The bubble transportation and breakup in the Venturi channel with the throat size of 1 mm × 2 mm shows similar characteristics with that in a conventional Venturi channel. Overall, Venturi with smaller size presents a better performance in producing fine bubbles.