The　unqualified　properties　of　interior　insulating　materials　as　the　main　insulation　of　high　voltage　DC　bushing　will　affect　the　operating　reliability　of　bushing.　In　this　paper,　two　kinds　of　insulating　materials,　oil-impregnated　paper　(OIP)　and　epoxy/fiberglass　(E/F)　composite,　had　been　prepared　as　the　typical　specimens　for　present　studies.　Firstly,　the　influences　of　the　moisture　content　in　the　paper　and　temperature　on　the　dielectric　properties　of　OIP　test　were　studied.　The　lab　testing　setup　with　DC　voltage　up　to　50kV　for　DC　electrical　ageing　was　established.　The　main　dielectric　parameters,　including　the　dielectric　loss　factor　and　insulation　resistance,　of　specimens　during　DC　ageing　were　measured.　The　relationships　between　the　dielectric　properties　of　specimens　and　aged　duration　had　been　investigated.　The　infrared　spectroscopic　analysis　and　microstructure　examination　of　aged　specimens　were　carried　out　also.　The　results　show　that　moisture　content　within　OIP　could　reflect　the　dielectric　loss　factor　and　insulation　resistance　effectively.　Moreover,　compared　with　un-aged　specimens,　the　microstructures　of　long-time　aged　specimens　have　a　significant　variation.　This　study　will　provide　the　experimental　data　and　assistances　for　improving　the　main　insulating　materials　for　high　voltage　DC　bushing　application.　©　2011　The　Institute　of　Electrical　Engineers,　Japan.