A new expression of the SN curve based on the concept of "weakest link" is proposed. It is a probabilistic model introducing a new parameter Nc, the number characteristic of cycles corresponding to the failure probability equal to 1. His confrontation with the terms of the curve most used SN, particular those Basquin, the Wöhler and the Stromeyer, the fatigue tests and martensite steels P22O 100C6 data provided errors of about 5% maximum for models Basquin, Wöhler and that proposed. Expression takes into account the volume of the Seine part of the test, which will help explain the number variations of cycles for a given level of stress. The search for a correlation relationship between the fatigue limit D and the breaking strength Rm will help prevent the fatigue life of the work-hardened materials.