Abstract: The S-N curve slope of the welded joints is substantially different from the conventional base metal. Tensile strength and fatigue limit are two fundamental factors that are taken into consideration by the S-N curve prediction model, which is based on the notion of entropy in physics and thermodynamics. This model assumes that the S-N curve contains two horizontal asymptote components. The developed model was utilized to fit and assess the pertinent experimental data based on the fatigue life data of Ti6Al4V welded joints, S355J2N welded joints, and aluminum steel dissimilar welded joints given in the literature. The results demonstrated that: Using dissimilar welded joints made of aluminum steel as an example, the slope of the inclined section of the S-N curve remained nearly constant while the fatigue strength of the joint with reinforcement decreased similarly to the welded joint without reinforcement; Employing S355J2N and Ti6Al4V as examples, the slope of the inclined section of the S-N curve depends on the fatigue limit of the joint when the tensile strength of the joint becomes identical under different welding processes; A comparison between this model and the conventional S-N curve assessment model revealed that it has a mathematical form with the traditional Basquin equation, indicating a deeper physical relevance since it incorporates two parameters: tensile strength and fatigue limit.