Comprehensive flow induced vibration tests have been conducted on an inline array. HCFC-123 is used as the test fluid. HCFC-123 stands for Hydrochlorofluorocarbon, whose chemical formula is CHCl2CF3. The critical flow velocities and two phase flow induced damping were compared with the steam-water two-phase flow test results. For that purpose, the same test equipment as steam water two-phase flow test one were used. The responses of three essentially isolated flexible cylinders at different depths within the array were simultaneously measured. Unsteady fluid forces were also measured. Tests were conducted corresponding to some thermal hydraulic conditions. First, two-phase flow test at high pressure (0.76Mpa, 98°C), homogeneous void fraction β = 0.90, was conducted. This condition can simulate the flow pattern of the steam water two phase flow (5.8MPa, 273°C) because the density ratio between gas and liquid is equal for both fluids. Second, a single-phase liquid flow test at atmospheric conditions was conducted. Finally, a single-phase flow test at high pressure (0.76Mpa) in HCFC-123 vapor was conducted. The characteristics of fluidelastic vibration were very close between HCFC-123 and steam-water two phase flow.