Abstract: The direct wave part is often applied in traditional ultrasonic detection method.However,limitations are encountered during the inspection of anisotropic and non-homogeneous structures.In this work,the ultrasonic wave propagation in such multiple scattering media is treated as a diffusion process.The surface defect localization with the ultrasonic diffuse field is explored.The diffuse wave field is sensitive to the defect through dense sampling of the medium.Regarding to the carbon fiber reinforced epoxy plate with 0°/90°₁₂ layer-up,a theoretical model is established based on the Locadiff method,from which the decorrelation analysis is realized with the defect sensitivity kernel.According to the measured waveforms before and after the defect appearance,both the diffusivity and decorrelation coefficients can be determined.The defect location can be achieved with an inversion algorithm finally.The results show that the minimum localization error is 6.99% for the defect of 10 mm in diameter while 9.29% for the defect of 5 mm in diameter.The diffuse wave is promising for the defect inspection in composites.