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Based on the expression of the dispersion equation of Lamb waves in an adhesive two-layered plate presented in our previous paper [Chin. Phys. Lett. 18(2001)1483], the two lowest Lamb modes, the symmetric mode So and the antisymmetric mode Ao are successfully decomposed in the low-frequency regime. Relations between the rigidity of the bond and the velocity of the two Lamb modes are found, which lay a foundation for the estimation of the bond rigidity of the adhesive plate. The influence of the variations of the bond rigidity in terms of the stiffness constants KN and KT of the spring model on the velocity of the two Lamb modes is discussed and numerically evaluated. Numerical results indicate that the deterioration of the bond rigidity causes the phase velocity decrease for Lamb modes of the two lowest order, thus having a possibility for the evaluation of the bonding state of the adhesive plate by using ultrasonic wave velocity measurement.