There has been a great concern about the on-line structural health monitoring in plate-like structures using guided wave tomography method. Most studies on guided wave tomography have only dealt with defect position. They cannot give information of defect size. In this paper, we studied tomography algorithm to detect a defect size using reflection of guided wave. The work presented here introduces a Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID) which is used to construct tomographic images of defect position and size. Through this study, a feasibility of quantitative defect imaging based on the guided waves tomography is explored.