To provide a method capable of further accurately evaluating durability of a spring using an X-ray stress measurement method.SOLUTION: The evaluation method of a spring evaluates durability thereof on the basis of a value of ψ split width calculated from a (2θ-sinψ) diagrammatic drawing. The ψ split width is calculated by the following steps of: first, defining a measurement point on a surface of the spring subjected to shot-peening and prescribing a Z axis and an X axis in a normal direction and a main stress direction respectively, of the surface of the spring passing through the measurement point; next, applying X ray to the measurement point from two directions on both sides of the Z axis at a plurality of angles ψ, and plotting a diffraction angle 2θ obtained about the angles ψ, with respect to a (sinψ) to create the (2θ-sinψ) diagrammatic drawing; and calculating differences of the diffraction angle 2θ with respect to any angles ψ on the basis of the created (2θ-sinψ) diagrammatic drawing and defining a maximum value of the differences as the ψ split width.SELECTED DRAWING: Figure 1
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