An immerged photoimaging system was developed to measure bubble size distribution, which mainly consisted of a fibre-optic endoscope, an image acquisition system and a light source. Bubbles generated by a microbubble generator and two ceramic microporous tubes with different pore-sizes were measured using this system, and the results were compared with those by the conventional sampling photoimaging method. The bubble sizes generated by the three generators ranged from 50 to 2000 mm. The difference between the two methods was not obvious when the bubble diameter was smaller than 200 mm. However, it became significant when the bubble diameter was larger than 200 mm. This is probably because smaller bubbles are relatively more stable, and bigger bubbles are easy to coalesce, leading to measurement errors in the sampling course. The results suggest that this new method is capable of measuring a wide range of bubble sizes reliably, and can be used for in-situ measurement of bubble size in gas-liquid systems, verification of other methods and numerical simulation.