In this paper, flow-induced vibration analysis and experiment of the thermocouple well sustained in cross flow of Japanese Fast Breeder Reactor Monju are performed. Navier-Stokes equations coupled with equations of structural motion are solved using the finite element method. The experiment is performed in water using real scale mock-up model. The in-line oscillation mode is simulated in both experiment and analysis and the results are consistent each other. Also observed in both the experiment and analysis are the symmetric vortices that are generated at both lateral sides of the cylinder in accordance with the cylinder natural frequency. Furthermore, it has been found both in numerical simulation and experiment that the thermocouple well oscillates at decreased flow rate if the natural frequency of the thermocouple is reduced. Based on the analytical and experimental results, a displacement response diagram that relates the displacement and the reduced velocity is established.