Due　to　the　superior　thermal　and　dielectric　properties　of　polypropylene(PP),　PP　becomes　a　suitable　candidate　for　thermoplastic　insulator　of　high-voltage　direct-current(HVDC)　cable.　In　this　paper,　both　nano-SiO2　and　polyethylene　octene(POE)　were　introduced　in　PP　matrix,　in　order　to　modify　the　crystalline　and　improve　the　electrical　and　mechanical　properties.　The　morphology,　DC　breakdown　strength,　space　charge　distribution,　crystallization　behavior　and　mechanical　properties　were　characterized　by　scanning　electron　microscopy(SEM),　polarized　light　microscopy　(PLM),　AC/DC　breakdown　system,　pulsed　electro-acoustic　system(PEA),　differential　scanning　calorimeter(DSC),　and　electronic　universal　testing.　The　results　showed　that　the　DC　breakdown　field　was　enhanced,　the　space　charge　was　inhibited　and　the　mechanical　properties　were　improved　in　the　composite　with　the　co-doping　of　nanoparticles　filling　and　POE　blending.　The　breakdown　strength　increased　by　about　25%　and　the　elongation　at　break　increased　by　about　30%　in　the　composite　compound　of　PP,　POE　and　nano-SiO2,　compared　with　raw　PP　sample.　The　modification　of　mechanical　and　dielectric　properties　provides　promising　results　for　the　application　of　PP　in　HVDC　cables.　©　2019,　High　Voltage　Engineering　Editorial　Department　of　CEPRI.　All　right　reserved.