Mode I fatigue crack growth behaviour was experimentally investigated under constant and following the application of a single spike overload. Different specimen thicknesses were investigated at different overload ratios. A previously developed crack tip deformation model to correlate fatigue crack growth behaviour following k  a single overload cycle was used to correlate that behaviour for different specimen thicknesses. It was found that the effect of specimen thickness on the fatigue crack growth behaviour under constant amplitude of loading is marginal especially at growth rates greater than le mm/cycle. The retardation due to the application of a tensile single overload decreases with the increase in specimen thickness. Good agreement between relevant experimental results and those predicted using three dimensional crack tip deformation parameter was found.