Thermowells with helical strakes are becoming promising to prevent them from fatigue fracture by Kármán vortex street. Many studies suggest various kinds of measurement techniques, including strain rate measurement, acceleration measurement, and high-speed visualization to evaluate the role of Kármán vortex street to the flow-induced vibration. Nevertheless, use of laser Doppler vibrometry has not yet been reported in the literature. This study compared the tip deflection of a thermowell due to the flow-induced vibration by using the laser Doppler vibrometry and the strain rate measurement. The laser Doppler vibrometry could measure the tip deflection directly. On the other hand, the strain rate measurement had to convert the strain rate into the tip deflection through the Euler-Bernoulli beam theory. Measurement equivalence between the laser Doppler vibrometry and the strain rate measurement was discussed with the results of tip deflections of the thermowell.