Abstract: Based on elastic wave propagation theory a dispersion equation for Lamb waves in a plate with one viscous liquid layer loading is derived.Viscosity-induced attenuation of the wave is discussed.Numerical calculation shows that resonance vibration of the liquid layer and the plate may cause the changing of wave modes and the variation of attenuation due to asymmetrical liquid layer loading.When k_td ≤ 1,mass and viscosity sensing application of A₀ mode Lamb wave is also discussed.