The leak before break (LBB) assessment guidelines for sodium cooled fast reactors (SFRs) is being developed in the Japan Society of Mechanical Engineers (JSME). The major purpose of the guidelines is to provide LBB assessment procedures for pipes and vessels of SFRs that retain sodium coolant. Design features of SFRs, such as high operation temperatures and low pressure coolant-systems, are taken into account. The LBB guidelines is used in connection to the fitness-for-service code that is also being developed for SFRs in JSME. If the establishment of LBB concept is successfully demonstrated, continuous leakage monitoring will be adopted as an in-service inspection for SFR components constituting sodium coolant boundary, such as sodium retaining pipe and vessels. The LBB assessment consists of three main paths, which are “Unstable fracture assessment”, “Leak rate assessment” and “Penetrated crack assessment”. “Unstable fracture assessment” is conducted to determine a critical crack length, ccr, which introduces unstable fracture under given loading conditions. “Leak rate assessment” is carried out to determine detectable crack length, cd, above which sodium leakage can be detected by installed leak detectors. In order to assess the cd, a new crack opening displacement evaluation method was developed for thin walled large diameter cylindrical structures taking the geometrical feature of SFR pipes into account. “Penetrated crack assessment” is performed to determine cp, the crack length when a crack penetrates the wall, under given transients hypothetically repeated enough cycles for the crack to penetrate the wall. In order to assess the cp, a simplified method was developed both for axial and circumferential cracks as a function of the ratio of membrane and bending stresses and fatigue crack growth characteristics. In the cp assessment, penetrated crack length is evaluated by considering only fatigue crack growth mechanism because penetrated crack length without considering creep effects becomes longer. If ccr is longer than both cd and cp, it can be concluded that the establishment of LBB concept is demonstrated. To demonstrate the procedure of LBB guidelines, actual SFRs piping was evaluated and the establishment of LBB concept is successfully demonstrated.