By using techniques developed by many workers to study streamer mechanism in air between metallic electrodes, the phenomena of partial discharges in insulating walls were studied. It is shown that the luminous evolution of partial discharges consisted of the development of a primary and a secondary streamer, for positive pulses. The velocity of propagation of the primary streamer was about 5 × 105m/s as for metallic electrodes. The electrically induced pulses and photon pulses have shown a correspondence between electrical and luminous phenomena for positive pulses, while Trichel pulses were observed for negative pulses. A cathodic zone was shown, and then a mechanism of erosion of insulating materials was conceived.