Abstract: Sometimes there is a totally thickness data error during the steel plate thickness measurement with an ultrasonic thickness gauge. The reasons caused the error were analyzed and found that ultrasonic thickness gauge had a high testing sensitivity to detect the material defects, such as tiny defect, lamination, material segregation, and material uneven, which all could affect the accuracy and resolution of thickness measurement easily. A suggestion was proposed to take on an ultrasonic testing on the abnormal location to preclude getting an error thickness data for the thickness measurement of the steel plate.