The ability to model and simulate guided wave propagation provides an insight into the development of robust guided wave inspection systems. This paper presents the finite-element method (FEM), as a powerful computational technique used in the context of ultrasonic guided wave nondestructive evaluation of inaccessible plate like structures. A commercially available FE package is used for this study. Corrosion type surface thinning with depths from 20% to 50% are fabricated in a steel plate. A series of computer runs are made using "Lamb type" guided waves and the results of the parametric study are then analyzed. Based on the analysis, a quantitative method to estimate average thickness of the plate is suggested. Experimental validation is also provided.