The in-plane (stream-wise) fluid-elastic instability of triangular tube arrays caused tube-to-tube wear indications as observed in the U-bend regions of tube the bundles of San Onofre Unit-3 steam generators. To understand the in-plane FEI characteristics, stability analysis using unsteady fluid forces is quite helpful. However, taking measurements of unsteady fluid forces in the in-plane direction is quite challenging as the fluid coupling forces of the surrounding tubes must be measured simultaneously for the in-plane stability analysis. In particular, taking measurements under a high pressure and high temperature steam-water conditions, similar to an SG operating condition, is extremely difficult. Recently we have been able to successfully measure unsteady fluid force using a specially designed test equipment. In the meantime, random excitation forces acting on heat transfer tubes were also measured. We calculated the stability boundary of the in-plane FEI using the unsteady fluid force measurement with this special equipment.