A good knowledge of the evolution process of a crack or cavity is necessary for a better understanding the mechanical behavior of the damage and failure of materials and structural components. Before damage begins, the size of the initial or prescribed cracks and cavities are usually very small. These small cracks and cavities can be conveniently used as the diffraction apertures of the coherent light. By analyzing the diffraction fields, the characteristic parameters of these special diffraction apertures can be obtained and, hence, the deformation fields. Experimental tests are conducted to demonstrate the reliability and accuracy of the laser diffraction technique in detecting the evolution and propagation of a small crack or cavity. The relationships between the coherent diffraction patterns and the crack and cavity geometric parameters are established for the simple shape of the crack or cavity aperture. Moreover, the autocorrelative method and the spectral iterative technique are introduced to retrieve the small crack and cavity apertures for the complex shapes.