In this paper, experimental evidence is presented that suggests a strong nonlinear interaction between acoustic wave and micro‐structural damage before the onset of delaminations in fatigued CFRP samples. Sample used were 32 plies quasi‐isotropic graphite/epoxy laminate fatigued with a four point bending fatigue. First harmonic images were constructed from the amplitude of the first harmonic normalized by the amplitude of the fundamental. Harmonic imaging technique (HIT) shows a much higher sensitivity to micro‐damage than amplitude C‐scan. Correlations are established between the image zone where the nonlinear parameter is high and the region where a high density of micro‐delamination and matrix cracks is observed.