In recent years, in a newly installed replacement steam generator, in-plane (IP) fluid elastic instability (FEI) for the heat transfer tubes has occurred. The fluid elastic instability is one of the severe vibrations in the heat transfer tube bundle and should be avoided. There have been many studies on the out-of-plane (OOP) fluid elastic instability, and the design evaluation guideline based on Connors' equation and the results of flow tests has been established. On the other hand, no evaluation guideline has been established for in-plane fluid elastic instability, and no critical coefficient has been determined in high-temperature, high-pressure steam–water two-phase conditions. Therefore, in this paper, in order to develop the guideline for evaluating in-plane fluid elastic instability, the critical coefficients were obtained using two types of test equipment for rotate triangular array in two-phase flow (SF6 ethanol) simulating steam–water flow under high-temperature and high-pressure conditions.