The technique of laser photoacoustic spectroscopy is applied to the studies of surface and sub-surface structures of solids in a nondestructive fashion. In the present case, special attention is focused on reaction-bonded Si N ceramics, which are used for the manufacturing of turbine blades. Good correlation is obtained between the observed photoacoustic signal and surface microstructures. In addition, the photoacoustic signal reveals inhomogeneities that are not visually detected under a microscope.