The paper presents a study for quantitative imaging of damage in metallic plates using the fundamental anti-symmetric mode of (A₀) Lamb wave. The study proposed a two-stage approach, in which the damage location is first determined in stage-one through analyzing the cross-correlation of the excitation pulse and scattered wave signal, and the damage is then characterized in stage-two using the Mindlin plate theory based Lamb wave diffraction tomography. The damage considered in this study is an elliptical shape of plate thickness reduction, which is a simplified representation of corrosion damage in metallic plates. The two-stage approach is employed to quantitatively image the plate thickness reduction, i.e. determine the location, size and shape of the thickness reduction. Finite element simulation of a circular transducer network with eight transducers is used to demonstrate the capability of the two-stage approach in characterizing the damage. The results show that the two-stage approach is able to accurately identify the damage location and provide a reasonable estimation of the size and shape of the damage.