Abstract: Gas-liquid two phase flow patterns were studied in two thin rectangular channels made of plexiglass. One of the channels is 1000mm in height, 140mm in width and 4mm in thickness, and the other 1000mm in height, 140mm in width and 8mm in thickness. Air-water was used as working media, which flow upward in the rectangular channels. Bubble flow, slug flow, churn flow and annular flow were found both by visual observation and by photography. They were basically similar to the flow patterns in a vertical circular tube. The flow pattern maps were plotted as 1/Xtt versus G, and Jg* versus RL. The former is similar to the flow pattern maps in a vertical circular tubeproposed by J. R. Fair. However, the ranges oi flow region are different. The following expression was obtained by regression of the experimental data The equation can be used to determine the transition region from one type of flow pattern to another. The coefficient K and the index n in the equation are related to the flow region and are dilferent in different flow regions. The pressure drop was measured by using a x-y functional register. The variation in the frequency and amplitude of pressure drop changes with different flow patterns. The pressure drop spectrum can be used to determine flow patterns qualitatively. The pressure drop of gas-liquid two phase flow in thin rectangular channels was calculated with the Lockhart-Martinelli method. The results obtained were compared with the experimental data.