Abstract: Viscosity is an important physical property of fluid and a quality index of many liquid industrial products. In lots of industrial product process, viscosity measurement is important means of controlling product flow, achieving product security, improving and guaranteeing quality of products, saving and developing energy. Ultrasonic test method is used widely in the test of industry because its character by which on-line test is easy to be realized. In this paper, feasibility of measuring using piezoeletric response voltage, which is produced with ultrasonic wave, is analysed in theory. And the relationship of viscosity and acoustic impedance, acoustic impedance and piezoeletric response voltage is obtained. From that, the corresponding relationship of viscosity and response voltage is proved on the certain temperature and certain frequency.