It has been known that the cold expansion repair technique is very beneficial for the fatigue life of damaged structures. A numerical investigation has been realised to analyse the optimum degree of cold expansion (DCE) to avoid the great number of fatigue test and to achieve the maximal fatigue lifetime. The numerical results have been validated by some experimental results of fatigue tests about steel and an aluminium alloy. This work proposes a method to determine the optimum degree of cold expansion for different materials using the finite element method. The high equivalent von Mises stresses around the expanded hole during the process of cold expansion can cause local damage. The position of instability, the ultimate stress corresponding to the point of instability on the true stress–true strain curve were used to determine the optimum degree of cold expansion.