<p>This study investigates the effect of z-pin through-thickness reinforcement on the Mode I interlaminar fracture toughness of composite laminates. Initially, experimental test data were used to validate multiscale Finite Element Analysis (FEA) models developed to simulate Double Cantilever Beam (DCB) mechanical evaluation of z-pin reinforced composites. The validated models were then employed to explore the enhancement in interlaminar toughness associated with variations in z-pin diameter and areal density. Results indicate that z-pins substantially improve fracture toughness, with smaller diameter z-pins and higher areal densities yielding the greatest enhancements as to be expected. This improvement is attributed to a greater number of active z-pins bridging the crack front and an expanded interfacial surface area. Furthermore, the study finds that the z-pin layout pattern exerts minimal influence on interlaminar performance, with improvements primarily driven by optimizing pin size and density. These findings provide theoretical support for the optimization of z-pinning techniques and their application in advanced composite structures.</p>