Abstract: The two-phase flow patterns of air-water mixture were studied in a narrow rectangular channel (40 mm×1.6 mm) in rolling and steady condition, respectively. The flow patterns were observed by using a CCD camera and were identified by examining the video images, and can be classified into bubbly flow, slug flow, churn flow and annular flow. The flow patterns were compared with the large diameter round tube, and the flow pattern maps were given. The effect of rolling on flow pattern in narrow rectangular channel is similar to that in the large diameter round tube, but due to the small room and surface tension effect.