Abstract: The high voltage ceramic post insulator of a 110 kV substation fractured. The fracture reasons were analyzed by means of macroscopic observation, porosity test, scanning electron microscopy and energy spectrum analysis, X-ray diffraction analysis and other methods. The results show that the improper control of the firing process caused the insulator to appear yellow core. The pore defects in the flange cement mounting parts and sudden changes in ambient temperature were also the reasons for the decrease of insulator strength.