Abstract: To investigate the air-water structural characteristic of the self-aerated developing zone in the open channel flow, the bubble size and quantity changes with the self-aeration development were evaluated experimentally, using a double-tip conductivity probe. The results showed that the distribution range of bubble chord length and mean chord length decrease gradually with reduced air concentration in the same flow cross-section; within the self-aerated zone, small-sized bubbles dominate quantitatively while large-sized bubbles have a large portion in terms of air content. It was believed from analysis that from the area at high air concentration near the water surface to the one at low air concentration at the bottom of the self-aerated zone, the development process for the air-water structure of self-aerated flow is predominant surface deformation-coexistence of surface deformation and individual bubbles-individual bubbles, which will be helpful for further study on formation mechanism of self-aeration in open channel flows.