Abstract: The purpose of this work is to measure bubble size distribution of bubbly flowsin the column by using image analysis technology. Firstly, images of bubbles for a range of superficial gas velocities are obtained by using high speed camera with lighting emitting diode(LED) as a light source. Air and water are used as gas-liquid bubbly flows. Then, the images are preprocessed and converted into binary imagesby employing Matlab. Thus,perimeters and areas as factors of roundnessare extracted to classify bubbles into overlapping and solitary bubbles.Solitary bubbles are separated to prevent the software segmenting them mistakenly. The size distribution of solitary bubbles is achieved by the use of formula. Thenthe overlapping bubbles are segmented into solitary bubbles by the use of an improved watershed algorithm.Finally, thesize distributionof all bubbles at different gas velocitiesisanalyzed and compared with the distribution without watershed.It shows that the size distribution with watershed is more accurate than the size distribution without watershed. The gas hold-up has also been calculated by the use of size distribution. In order to check the accuracy of the result,the calculated gas hold-up and actual gas hold-up have been compared and the measurement error between those twois expressedin the form of figure. The error grows with the increase of superficial gas velocity, less than 10% at1.2~1.8cm·s-1.But the size distribution obtained by the image analysis technology cannot describe the actual gas hold-up when the superficial gas velocity is more than 2.0 cm·s-1. In spite of this, the size distribution detected can help us to learn more rules of bubble motion. For example, the growth trend of the bubble’s interfacial area slows down with the increase of superficial gas velocity. And the resulting bubble size distribution with the superficial velocity between 1.2cm·s-1 and 2.0 cm·s-1isshown in the form of probability density function(PDF)