Abstract: Visualized experimental observation on flow patterns during the flow boiling of water under single-side heated and fluid-inlet sub-cooled conditions in a vertical narrow rectangular channel with the section of 40×3mm² were carried out.Four discernible flow patterns which were dispersed bubbly,and the coalesced bubbly,churn flow and annular flow were observed.Flow visualization in two dimensions of two-phase flow patterns for narrow rectangular channel,which provided intuitionistic evidence to distinguish flow patterns,were performed.Based on the experiment results,a flow pattern map for the single-side heated narrow rectangular channel was developed and then compared with some existing flow pattern maps.The comparison result showed that the flow patterns and its transition mechanisms are obviously different between the heated steam-water flows and the adiabatic air-water flows.