In recent years, there has been a growing need for weight reduction of automotive suspension springs for the purpose of global environment protection, and with this, high-strength springs have been increasingly required. However, delayed fracture is induced when springs become highly-strengthened. Therefore development of a design method considering delayed fracture and of prevention techniques is an urgent issue. In order to realize the design method for springs that prevents delayed fracture, this report provides the database of fracture mechanics parameters, i.e., threshold stress intensity factors of hydrogen embrittlement for spring steel (hereafter referred to as KIHE). To be specific, the relationship between KIHE and hardness of SUP9A, SUP10, SUP11 and 51CrMoV4 steels, which are frequently used for leaf springs, was examined by the Wedge Opening Loading (WOL) testing. To confirm the usefulness of the relationship, the delayed fracture testing by four-point bending was conducted on test specimens with a semicircular notch. Moreover, the effectiveness of shot peening to prevent delayed fracture, as a prevention technique, was also demonstrated, and the design method considering residual streess to prevent delayed fracture was studied as well. The acquired parameters for the prevention of delayed fracture can be applied not only to leaf spring steels, but also to spring steels in general.