Ferromagnetic materials have the interaction between mechanical and magnetic fields, which is called magnetoelastic coupling. Also it is known that magnetic properties of ferromagnetic materials depend on plastic deformation. There are various kinds of Non-Destructive Testing (NDT) based on these phenomena for fatigue estimation. We propose a new analytical method of magnetic hysteresis curve, in which the saturation magnetic hysteresis curve is decomposed to neutral magnetization curve and hysteresis gap curve. Each of the curves is expanded into power series, and its coefficients are extracted as parameters for the estimation. To verify this method, saturation magnetic hysteresis curves of STS410 steel with various fatigue degrees are measured. The method is found to be promise for the fatigue estimation.