Abstract: In order to systemically investigate the effects of nano SiO₂ on the alternating current/direct current (AC/DC) breakdown strength and AC/DC electrical treeing resistance properties of cross-linked polyethylene (XLPE), nano SiO₂/XLPE composites with 0.5wt% and 1wt% nano SiO₂ were prepared using parallel twin screw extrude. With untreated XLPE and commercial DC cable XLPE insulation as references, the doping effects of nano SiO₂ on the electrical tree initiation and growth characteristics under AC voltage and periodic grounded DC voltage as well as the AC/DC breakdown strength of XLPE were tested. The experimental results indicate that the DC breakdown strength of commercial DC cable XLPE insulation is similar to that of the untreated XLPE, but the periodic grounded DC electrical tree initiation is more difficult and the tree growth speed is much slower than that of untreated XLPE. With the increase of nano SiO₂ content, its effect on improving AC/DC breakdown strength and inhibiting AC/DC electrical tree initiation are increased. The 1wt% nano SiO₂/XLPE composite has the significant inhibition effect on the imitation and growth of the periodic grounded DC electrical tree, and its tree initiation and growth properties are both superior to the commercial cable material. The AC breakdown strength and the initiation voltage of AC electrical tree of 1wt% nano SiO₂/XLPE composite are both higher than that of untreated XLPE, but the inhibition on the growth of AC electrical tree is limited to the early stage of tree propagation. After the electrical tree exceeds a certain length, the growth rate of electrical tree in the 1wt% nano SiO₂/XLPE composite exceeds that of untreated XLPE.