In order to perform monitoring of the polymerisation process, it is necessary to measure viscosity. However, in the case of non-Newtonian highly viscous fluids, viscosity starts to be dependent on the vibration or rotation frequency of the sensing element. The objective of the presented work is to develop ultrasonic method for measurement of viscosity of high viscous substances during manufacturing process. For this purpose, the method based on application of guided symmetrical Lamb waves propagating in an aluminium planar waveguide immersed in a viscous liquid has been proposed. In addition, theoretical and experimental validation has been performed.