Abstract: Taking deionized-water as working fluid, the flow pattern transformation process in a three-side heating narrow rectangular channel with a 3 mm×43 mm cross section was studied with visualization experiment. A high speed camera was used to record visualization data. Four main flow patterns, bubble flow, slug flow, stirring flow and gas film plug flow, were observed, and the transformation process of bubble forms inside channels of various flow patterns was recorded in detail. The critical point of the flow pattern transformation was recorded and the flow patterns of three-side heating narrow rectangular channel were drawn. The trend and formation mechanism of the boundary curves of flow pattern transformation in flow pattern map were analyzed. Compared with the existing flow patterns of similar channel size, it shows that the flow pattern transformation process and the trend of some flow pattern transformation curves are very different between the three-side heating flows and the adiabatic air-water flows. Due to the effect of narrow-side heating section, the flow pattern transformation process is also significantly different from that in single-side heating channel. The gas film slug flow does not appear either under the adiabatic or the single-side heating conditions.