HVDC cables have shown great advantages for long-distance, high-power underground or underwater transmission. The performance of cable accessories, as an essential part of the HVDC networks, is a great concern to the reliability of the system. However, Cable accessories made of ethylene-propylene-diene terpolymer (EPDM), which are considered to be the weakest part of HVDC cable system, have to face the problem of space charge accumulation. Nanosized particles have been proved as an effective method to suppress the space charge accumulation in dielectric composites. This chapter presents a study aimed at clarifying the effect of nanoparticles and direct-fluorination on space charge behaviors for HVDC cable accessory insulation. Obtained results show that the dielectric properties and DC conduction for HVDC cable accessory insulation are significantly influenced and the interface charge density can be effectively suppressed by doped fillers.