Abstract: In this paper, the flow patterns of the two-phase flow induced by flow flashing were studied by using the high-speed photography and wire-mesh sensor in the vertical riser of a low-pressure natural circulation experimental loop. For different flow states of the natural circulation system, the types of flow patterns were presented on the conditions of stable and unstable states, respectively. Furthermore, on basis of the axial fluid temperature distribution, the location of sharp temperature drop was determined as the start point of flashing. Two-phase flow stability region can be determined by the inlet subcooling number and flashing number. When the inlet subcooling number is less than 12 and flashing number is between 4 and 5, the two-phase natural circulation flow induced by flashing is stable.