Abstract: A visualized study of flow patterns was carried out for air-water two-phase flow in two narrow rectangular ducts (40 mm×1.41 mm, 40 mm×10 mm) respectively. The superficial velocity ranges of air and water were 0.01-0.59 m/s and 0.02-3.72 m/s. Based on three classical criteria on transition from bubbly flow to slug flow, critical transition void fraction 0.23 was deduced by introducing the effect of channel size. The rectangular ducts were classified as small channel (s≤2.5 mm) and conventional channel (s>2.5 mm), based on which a modified transition criterion for bubbly flow to slug flow was proposed. The modified criterion shows a good agreement with the experimental data from this study. Experimental data from Mishima, Wilmarth and Sadatomi etc. were also collected to further evaluate the accuracy and applicability of the modified criterion, showing a good prediction to these experimental data as well.