Abstract: The fluid-elastic instability of heat-transfer tubes is a significant reason that causes tube failures in steam generators. Whether the fluid-elastic instability of a heat-transfer tube occurs depends on its modal frequencies. In the actual structure, the in-plane supporting is supplied for the U-band region of the heat transfer tubes by the anti-vibration bars (AVBs) subjected to friction, and is different from the out-of-plane one in supporting mechanism and direction. This makes the setting of boundary condition at supporting points very important in the modal analysis. In this paper, the out-of-plane boundary conditions were set as simply supported, however, springs were used to simulate the inplane supporting subjected to the friction between the tubes and AVBs. By changing the stiffness of the springs, the effect of in-plane supporting magnitude on the flow-induced vibration of heat-transfer tubes was discussed. It is indicated that the in-plane fluid-elastic instability may occur earlier than the out-of-plane instability when the contact stiffness between the tube and AVBs is loose.