Abstract: The aim of this article was to emphasize the importance of the display-response-time in order to attract the necessary attention from manufacturers and users of digital ultrasonic instruments. By comparison of echo-waves display principles between analogue and digital ultrasonic instruments, as well as the definition of display-response-time in European Standard(ES) EN 12668-1, this article explained the reason of display-response-time under digital ultrasonic instrument context, and the concepts and applications of maximum-effective-repeat-frequency. It also pointed out the deficiency of the measuring method under ES definition, and suggested examining the alarm. Moreover, this article introduced some simple methods to measure display-response-time, either by external testing-device, or by rough self-test method.