Abstract: The S-N curve slope of the welded joints is substantially different from that of the conventional base material. Based on the definition of entropy in physics and thermodynamics, a S-N curve prediction model was introduced. This model took into account two fundamental parameters: tensile strength and fatigue limit. It was assumed that the S-N curve contained two horizontal asymptote components. The developed model was utilized to fit and assess the pertinent experimental data regarding fatigue life of aluminum steel dissimilar welded joints, S355J2N welded joints, and Ti6Al4V welded joints given in the literature. The results demonstrated that with aluminum steel dissimilar welded joints as an example, the slope of the inclined section of the S-N curve remains nearly constant while the fatigue strength of the joint without weld reinforcement decreases similarly to the welded joint with weld reinforcement. With S355J2N and Ti6Al4V welded joints 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 evaluation model reveals that it has a similar mathematical form to the traditional Basquin equation, indicating a deeper physical relevance since it incorporates two parameters: tensile strength and fatigue limit.