Prototype Fast Breeder Reactor (PFBR) is a 500 MWe liquid metal cooled fast breeder reactor under construction at Kalpakkam, India. The core of PFBR consists of 1758 subassemblies supported at the bottom on the grid plate sleeves. Liquid sodium is used as the coolant and it flows axially from bottom of the subassembly to top and it is in highly turbulent regime. This turbulent flow can excite flow-induced vibration of fuel subassembly which can cause failure of the fuel pin clad tubes from fatigue, wear and vibration induced fretting. Excessive vibration of fuel subassembly can also results in reactivity noise, fatigue or rattling. Flow induced vibration studies of dummy fuel subassemblies in water were conducted in subassembly test facility and the design was qualified for PFBR. However it is planned to measure the amplitude and frequency of vibration during pre-commissioning tests of PFBR. Measurements are planned during the isothermal run of PFBR at 200°C with dummy subassemblies loaded in the core. Since measurement has to be carried out in high temperature sodium environment, non-contact measurement technique using ultrasound waves was developed and a device named SONAR was manufactured for PFBR. Extensive testing was carried out to assess the performance of the device for reactor application. This paper discusses the details of the FIV measurements on PFBR fuel subassemblies, details of ultrasonic technique and SONAR device, its testing and results and conclusion. © Krishtel eMaging Solutions Pvt. Ltd.