Abstract: A new laser ultrasonic sensor for testing samples' thickness is developed.The detection principle is discussed in detail.This sensor has high-frequency response,high sensitivity and is easy to adjust.The sensor is used to detect laser-induced ultrasonic longitude-wave at the epicenter of the rear metallic surface when pulse Nd:YAG laser is focused on polished iron plate.Well-defined longitude-wave and its echo waves are acquired.Based on the experimental results,the sample's thickness can be deduced.The theoretical and experimental results are in good agreement with relative error of 0.5%.