The effects of overload on the threshold stress intensity factor of delayed fracture (KIHE) for spring steel were studied. Firstly, tensile overload was applied to a wedge engraved SUP9 specimen (Wedge Opening Load; WOL specimen), then delayed fracture tests were carried out to determine the resultant KIHE. It developed that the value of KIHE increased together with the stress intensity factor KOV by applying tensile overload. The effects of tensile overloading on KIHE were evaluated based on the theory of fracture mechanics. The KIHE correlates with the KOV, and the predicting equation has been established.