Composite insulators aged in-service for almost 10 years at the transmission line of 230 kV, in an area where brittle fracture of a composite insulator occurred, were inspected by the micro-computed tomography, µ-CT, and by the scanning electron microscope, SEM. As a result, several cracks with different depths under pollutant scale and confined in silicone housing were detected by the µ-CT. Surface of silicone housing was observed by the SEM. Many randomly oriented cracks from micrometers size were observed. However, larger cracks detected and evaluated by the µ-CT appear essentially perpendicular to the composite rod axis. In order to explain the evolution of those large cracks in composite insulators, a model based on µ-CT and SEM data was proposed.