An ultrasonic spectral analysis based methodology has been developed and successfully implemented to measure wall thickness in the range of 0.3 to 0.7 mm in iron thimbles using low frequency ultrasonic waves (around 10 MHz). Conventional ultrasonic techniques cannot be used in such cases because of the low thickness of the thimble and the presence of curvature. Using a low frequency (15 MHz) immersion transducer and low digitization (under 200 MHz), an accuracy of ±10 μm has been achieved. The importance of the window for selection of time domain data for spectral analysis has been clearly brought out. A software program has been developed for automated thickness measurement across a line scan and for displaying the surface plot of thickness variations using the developed methodology.