The　growth　characteristics　of　electrical　trees　in　XLPE　cable　insulation　under　DC　voltage　conditions　were　studied　by　a　combination　of　treeing　test　and　microscopic　observation　system.　In　the　treeing　test　samples,　the　tip　radius　of　the　needle　electrode　was　5　mu　m,　and　the　pin-plane　distance　was　2　mm.　When　a　70　kV　DC　voltage　was　applied　on　the　samples　for　totally　10　hours,　either　continuously　or　intermittently,　no　trees　appeared.　So　the　trees　for　growth　study　were　all　initially　triggered　by　a　50　kV　standard　lightning　impulse.　The　DC　voltages　were　applied　periodically,　so　as　to　make　it　possible　for　the　required　observation　and　recording.　Under　the　DC　voltages,　the　electrical　trees　developed　gradually　from　a　single　slender　branch　to　a　sparse　bushlike　structure,　and　tree　channels　were　found　to　be　non-conductive.　The　growth　rates　were　rather　slow　compared　with　AC　trees　under　the　same　condition.　Grouping　experiments　showed　that　the　tree　length　increased　with　the　increasing　number　of　cycles　and　amplitude　of　DC　voltage,　and　it　was　also　related　to　the　voltage　duration　in　each　cycle　at　60　kV.　This　tree　growth　pattern　has　been　well　explained　by　a　non-conductive　tree　model　and　the　equivalent　circuit,　and　the　discharges　occurred　in　the　tree　channels　are　thought　to　be　the　driving　force　for　tree　growth.　Under　the　condition　of　an　unexpected　grounding　of　the　needle　electrode,　a　new　tree　would　initiate　instantly　from　the　needle　tip,　while　an　existed　tree　would　grow　very　rapidly.　This　is　correlated　to　the　material　damage　caused　by　the　sudden　release　of　the　injected　charges.　The　work　is　useful　for　the　recommendations　on　test　arrangement　and　operation　strategy　of　HVDC　cables.