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Investigating the mechanical properties of harmonic structures during various stages of deformation, particularly after fracture, is critical. Herein, nanoindentation is used to evaluate the local deformation and fracture behavior of SUS304L steel harmonic structures. Electron backscattering diffraction is employed to observe the strain distribution in the fracture‐deformed samples, where high kernel average misorientation is evident near the shell–core boundary region. Comparison of the deformed sample reveals that the nanohardness of the shell and core regions significantly increases after deformation. This phenomenon indicates the capability of strain hardening. Furthermore, plastic inhomogeneity is observed before the fracture occurs. Strain‐induced α′‐martensite is observed in the fractured area, especially in the core region near the shell–core boundary, because of the high strain. High nanohardness is evident due to the high dislocation density and formation of strain‐induced α′‐martensite. The resulting high stress concentration can lead to void formation and crack initiation originating from the region near the shell–core boundary. [ABSTRACT FROM AUTHOR]