Overloads and underloads perturb steady state fatigue crack growth conditions and affect the growth rates by retarding or accelerating the growth. Clear understanding of these transient effects is important for the reliable life prediction of a component subjected to random loads. We can imagine that a random loading has a mean stress on a mean magnitude which can be used to determinate the baseline fatigue crack growth rate. The main objective of this research is to analyse the underload effect in fatigue crack propagation after an overload cycle.