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) and a circular tube with a diameter of 25 mm, respectively. The superficial velocity air and water were in the range of 0.03-24.71 m/s and 0.03-3.73 m/s. All the bubble, slug, churn and annular flows occurred in the three channels. Characteristics of flow patterns in the 40 mm×10 mm duct are very similar to those in circular tube, but different from the 40 mm×1.41 mm duct. Detailed flow regime maps of the three channels were presented. The experimental results show that the channel size has a significant influence on flow pattern transition, boundary of which in rectangular channels tends asymptotically towards that in the circular tube with increasing the width of narrow side.