The problem of damage induced stiffness degradation in composite laminates has been addressed by many approaches ranging from micromechanics to continuum damage mechanics. Most of these approaches are for design purposes but are not useful for inspection of structures during their service life. The damage in the form of delamination between plies of laminates is of particular interest for assessing the continued safety of structures. The current work aims to develop a methodology for quantifying the severity of delamination using Lamb waves as a means for structural health monitoring (SHM). Such SHM methodology will complement the design methods based on degradation in the material’s stiffness. In this study, the fundamental Lamb wave modes in their symmetric (S0) and anti-symmetric (A0) forms are used to establish a correlative measure of the delamination size present in the laminate. For this purpose, the interactions of the S0 and A0 modes with delamination are studied separately. As these waves pass through a damaged medium, the S0 modes produce features that agree with the in-plane stiffness, while the A0 mode is found to have characteristics corresponding to the out-of-plane stiffness of the composite laminate. The received wave signals with components representative of the delamination size are then post-processed using a statistical analysis to identify certain metrics developed previously as stress wave factors (SWFs). The evaluated SWFs are shown to have good correlation with the in-plane and out-of-plane stiffness degradation of the laminate that in turn depend on the delamination size.