The purpose of this paper is to describe the effects of liquid viscosity on ultrasonic propagation in a liquid/solid structure. Two types of propagation system were assumed: one was the solid/liquid structure, and the other was the solid/liquid/solid structure. In the former case, leaky SAW propagating along the interface was perturbed by the shear stress caused by the liquid viscosity. In the latter case guided waves propagating in the liquid layer were influenced by viscosity if the viscous penetration depth was comparable with the liquid thickness.