Because　partial　discharge　(PD)　in　air-filled　void　embedded　in　the　insulation　system　can　cause　the　degradation　and　eventual　insulation　failure,　it＇s　necessary　to　study　the　PD　mechanism.　Our　paper　employed　the　equipment　based　on　Pockels　effect　to　investigate　the　change　of　surface　charges　distribution　in　the　process　of　PD　degradation.　During　PD　sequences,　the　effect　of　previous　discharge　on　subsequent　discharge　was　also　examined.　©　2011　The　Institute　of　Electrical　Engineers,　Japan.